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CONSTITUTION  OF  THE  ATOM
CONSTITUTION  OF  THE  ATOM


WE  have  seen  that  whether  we  produce  the corpuscles  by  cathode  rays,  by  ultra-violet  light,  or from  incandescent  metals,  and  whatever  may  be the  metals  or  gases  present  we  always  get  the  same kind  of  corpuscles.  Since  corpuscles  similar  in  all respects  may  be  obtained  from  very  different  agents and  materials,  and  since  the  mass  of  the  corpuscles is  less  than  that  of  any  known  atom,  we  see  that the  corpuscle  must  be  a  constituent  of  the  atom  of many  different  substances.  That  in  fact  the  atoms of  these  substances  have  something  in  common.
[[Thomson 1904/Chapter 5]]
 
We  are  thus  confronted  with  the  idea  that  the atoms  of  the  chemical  elements  are  built  up  of  sim- pler systems ;  an  idea  which  in  various  forms  has been  advanced  by  more  than  one  chemist.  Thus Prout,  in  1815,  put  forward  the  view  that  the atoms  of  all  the  chemical  elements  are  built  up  of atoms  of  hydrogen ;  if  this  were  so  the  combining weights  of  all  the  elements  would,  on  the  assump-
 
 
CONSTITUTION  OF  THE  ATOM  91
 
tion  that  there  was  no  loss  of  weight  when  the atoms  of  hydrogen  combined  to  form  the  atom  of some  other  element,  be  integers ;  a  result  not  in  ac- cordance with  observation.  To  avoid  this  discrep- ancy Dumas  suggested  that  the  primordial  atom might  not  be  the  hydrogen  atom,  but  a  smaller atom  having  only  one-half  or  one-quarter  of  the mass  of  the  hydrogen  atom.  Further  support  was given  to  the  idea  of  the  complex  nature  of  the atom  by  the  discovery  by  Newlands  and  Mende- leeff  of  what  is  known  as  the  periodic  law,  which shows  that  there  is  a  periodicity  in  the  properties of  the  elements  when  they  are  arranged  in  the  or- der of  increasing  atomic  weights.  The  simple  rela- tions which  exist  between  the  combining  weights of  several  of  the  elements  having  similar  chemical properties,  for  example,  the  fact  that  the  combin- ing weight  of  sodium  is  the  arithmetic  mean  of those  of  lithium  and  potassium,  all  point  to  the conclusion  that  the  atoms  of  the  different  elements have  something  in  common.  Further  evidence  in the  same  direction  is  afforded  by  the  similarity  in the  structure  of  the  spectra  of  elements  in  the  same group  in  the  periodic  series,  a  similarity  which  re- cent work  on  the  existence  in  spectra  of  series  of lines  whose  frequencies  are  connected  by  definite
 
 
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numerical  relations  has  done  much  to  emphasize and  establish ;  indeed  spectroscopic  evidence  alone has  led  Sir  Norman  Lockyer  for  a  long  time  to advocate  the  view  that  the  elements  are  really compounds  which  can  be  dissociated  when  the circumstances  are  suitable.  The  phenomenon  of radio-activity,  of  which  I  shall  have  to  speak  later, carries  the  argument  still  further,  for  there  seems good  reasons  for  believing  that  radio-activity  is due  to  changes  going  on  within  the  atoms  of  the radio-active  substances.  If  this  is  so  then  we must  face  the  problem  of  the  constitution  of  the atom,  and  see  if  we  can  imagine  a  model  which has  in  it  the  potentiality  of  explaining  the  re- markable properties  shown  by  radio-active  sub- stances. It  may  thus  not  be  superfluous  to  con- sider the  bearing  of  the  existence  of  corpuscles  on the  problem  of  the  constitution  of  the  atom ;  and although  the  model  of  the  atom  to  which  we  are led  by  these  considerations  is  very  crude  and  im- perfect, it  may  perhaps  be  of  service  by  suggesting lines  of  investigations  likely  to  furnish  us  with  fur- ther information  about  the  constitution  of  the atom.
 
 
CONSTITUTION  OF  THE    ATOM  93
 
The  Nature  of  the  Unit  from  which  the  Atoms are  Built  Up
 
Starting  from  the  hypothesis  that  the  atom is  an  aggregation  of  a  number  of  simpler  systems, let  us  consider  what  is  the  nature  of  one  of these  systems.  We  have  seen  that  the  cor- puscle, whose  mass  is  so  much  less  than  that  of  the atom,  is  a  constituent  of  the  atom,  it  is  natural  to regard  the  corpuscle  as  a  constituent  of  the  primor- dial system.  The  corpuscle,  however,  carries  a definite  charge  of  negative  electricity,  and  since with  any  charge  of  electricity  we  always  associate an  equal  charge  of  the  opposite  kind,  we  should expect  the  negative  charge  on  the  corpuscle  to  be associated  with  an  equal  charge  of  positive  electri- city. Let  us  then  take  as  our  primordial  system  an electrical  doublet,  with  a  negative  corpuscle  at  one end  and  an  equal  positive  charge  at  the  other,  the two  ends  being  connected  by  lines  of  electric  force which  we  suppose  to  have  a  material  existence. For  reasons  which  will  appear  later  on,  we  shall suppose  that  the  volume  over  which  the  positive electricity  is  spread  is  very  much  larger  than  the volume  of  the  corpuscle.  The  lines  of  force  will therefore  be  very  much  more  condensed  near  the
 
 
94
 
 
ELECTRICITY    AND    MATTER
 
 
corpuscle  than  at  any  other  part  of  the  system,  and therefore  the  quantity  of  ether  bound  by  the  lines of  force,  the  mass  of  which  we  regard  as  the  mass of  the  system,  will  be  very  much  greater  near  the corpuscle  than  elsewhere.  If,  as  we  have  sup- posed, the  size  of  the  corpuscle  is  very  small  com- pared with  the  size  of  the  volume  occupied  by  the positive  electrification,  the  mass  of  the  system  will practically  arise  from  the  mass  of  bound  ether close  to  the  corpuscle ;  thus  the  mass  of  the  sys- tem will  be  practically  independent  of  the  position of  its  positive  end,  and  will  be  very  approximately the  mass  of  the  corpuscles  if  alone  in  the  field. This  mass  (see  page  21)  is  for  each  corpuscle
 
equal  to  — ,  where  e  is  the  charge  on  the  corpuscle
 
and  a  its  radius — a,  as  we  have  seen,  being  about 10-13  cm.
 
Now  suppose  we  had  a  universe  consisting  of an  immense  number  of  these  electrical  doublets, which  we  regard  as  our  primordial  system  ;  if  these were  at  rest  their  mutual  attraction  would  draw them  together,  just  as  the  attractions  of  a  lot  of little  magnets  would  draw  them  together  if  they were  free  to  move,  and  aggregations  of  more  than one  system  would  be  formed.
 
 
CONSTITUTION  OF  THE    ATOM  95
 
If,  however,  the  individual  systems  were  orig- inally moving  with  considerable  velocities,  the  rel- ative velocity  of  two  systems,  when  they  came near  enough  to  exercise  appreciable  attraction  on each  other,  might  be  sufficient  to  carry  the  sys- tems apart  in  spite  of  their  mutual  attraction.  In this  case  the  formation  of  aggregates  would  be postponed,  until  the  kinetic  energy  of  the  units had  fallen  so  low  that  when  they  came  into collision,  the  tendency  to  separate  due  to  their relative  motion  was  not  sufficient  to  prevent  them remaining  together  under  their  mutual  attraction.
 
Let  us  consider  for  a  moment  the  way  in  which the  kinetic  energy  of  such  an  assemblage  of  units would  diminish.  We  have  seen  (p.  68)  that  when- ever the  velocity  of  a  charged  body  is  changing the  body  is  losing  energy,  since  it  generates electrical  waves  which  radiate  through  space,  car- rying energy  with  them.  Thus,  whenever  the units  come  into  collision,  i.e.,  whenever  they come  so  close  together  that  they  sensibly  acceler- ate or  retard  each  other's  motion,  energy  will  be radiated  away,  the  whole  of  which  will  not  be absorbed  by  the  surrounding  units.  There  will thus  be  a  steady  loss  of  kinetic  energy,  and  after a  time,  although  it  may  be  a  very  long  time,  the
 
 
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kinetic  energy  will  fall  to  the  value  at  which  aggre- gation of  the  units  into  groups  of  two  will  begin ; these  will  later  on  be  followed  by  the  formation of  aggregates  containing  a  larger  number  of  units. In  considering  the  question  of  the  further  ag- gregation of  these  complex  groups,  we  must  re- member that  the  possibility  of  aggregation  will depend  not  merely  upon  the  velocity  of  the  aggre- gate as  a  whole,  i.e.,  upon  the  velocity  of  the centre  of  gravity,  but  also  upon  the  relative  ve- locities of  the  corpuscles  within  the  aggregate.
 
Let  us  picture  to  ourselves  the  aggregate  as,  like the  ^Epinus  atom  of  Lord  Kelvin,  consisting  of  a sphere  of  uniform  positive  electrification,  and  ex- erting therefore  a  radial  electric  force  proportional at  an  internal  point  to  the  distance  from  the  centre, and  that  the  very  much  smaller  negatively  electri- fied corpuscles  are  moving  about inside  it.  The  number  of  corpus- cles is  the  number  of  units  which had  gone  to  make  up  the  aggre- gate, and  the  total  negative  elec- trification on  the  corpuscles  is
 
Fio.  15
 
equal  to  the  positive  electrifica- tion on  the  sphere.  To  fix  our  ideas  let  us  take the  case  shown  in  Fig.  15  of  three  corpuscles
 
 
CONSTITUTION    OF    THE    ATOM  97
 
A,  B,  C,  arranged  within  the  sphere  at  the  corners of  an  equilateral  triangle,  the  centre  of  the  triangle coinciding  with  the  centre  of  the  sphere.  First suppose  the  corpuscles  are  at  rest ;  they  will  be  in equilibrium  when  they  are  at  such  a  distance from  the  centre  of  the  sphere  that  the  repulsion between  the  corpuscles,  which  will  evidently  be radial,  just  balances  the  radial  attraction  excited on  the  corpuscles  by  the  positive  electrification  of the  sphere.  A  simple  calculation  shows  that  this will  be  the  case  when  the  distance  of  the  corpuscle from  the  centre  is  equal  to  .57  times  the  radius  of the  sphere.  Next  suppose  that  the  corpuscles,  in- stead of  being  at  rest,  are  describing  circular  orbits round  the  centre  of  the  sphere.  Their  centrifugal force  will  carry  them  farther  away  from  the  centre by  an  amount  depending  upon  the  speed  with which  they  are  rotating  in  their  orbits.  As  we increase  this  speed  the  distance  of  the  corpuscles from  the  centre  of  the  sphere  will  increase  until at  a  certain  speed  the  corpuscles  will  reach  the surface  of  the  sphere ;  further  increases  in  speed will  cause  them  first  to  rotate  outside  the  sphere and  finally  leave  the  sphere  altogether,  when  the atom  will  break  up.
 
In  this  way  we  see  that  the  constitution  of  the
 
 
QO  ELECTRICITY    AND    MATTER
 
t7O
 
aggregate  will  not  be  permanent,  if  the  kinetic energy  due  to  the  velocity  of  the  corpuscles  inside the  sphere  relative  to  the  centre  of  the  sphere  ex- ceeds a  certain  value.  We  shall,  for  the  sake  of brevity,  speak  of  this  kinetic  energy  of  the  cor- puscles within  the  atom  as  the  corpUBfatfar  tem- perature of  the  atom,  and  we  may  express  the preceding  result  by  saying  that  the  atom  will  not be  stable  unless  its  corpuscular  temperature  is below  a  certain  value.
 
We  must  be  careful  to  distinguish  between  cor- puscular temperature,  which  is  the  mean  kinetic energy  of  the  corpuscles  inside  the  atom,  and  the molecular  temperature,  which  is  the  mean  kinetic energy  due  to  the  motion  of  the  centre  of  gravity of  the  atom.  These  temperatures  are  probably  not in  any  very  close  relationship  with  each  other. They  would  be  proportional  to  each  other  if  the  law known  as  the  law  of  equipartition  of  energy  among the  various  degrees  of  freedom  of  the  atom  were  to apply.  This  law  is,  however,  inconsistent  with  the physical  properties  of  gases,  and  in  the  proof  given of  it  in  the  kinetic  theory  of  gases,  no  estimate  is given  of  the  time  required  to  establish  the  state  con- templated by  the  law ;  it  may  be  that  this  time  is  so long  that  gases  are  never  able  to  get  into  this  state.
 
 
CONSTITUTION    OF    THE    ATOM  99
 
Let  us  now  take  the  case  of  two  aggregations, A  and  £,  whose  corpuscular  temperatures  are  high, though  not  so  high,  of  course,  as  to  make  A  and  B unstable  when  apart,  and  suppose,  in  order  to  give them  the  best  possible  chance  of  combining,  that the  centres  of  gravity  of  A  and  B  when  quite close  to  each  other  are  at  rest,  will  A  and  B unite  to  form  a  more  complex  aggregate  as  they would  if  the  corpuscles  in  them  were  at  rest  ?  We can  easily,  I  think,  see  that  they  will  not  necessa- rily do  so.  For  as  A  and  B  approach  each  other, under  their  mutual  attractions,  the  potential  en- ergy due  to  the  separation  of  A  and  B  will  dimin- ish and  their  kinetic  energy  will  increase.  This  in- crease in  the  kinetic  energy  of  the  corpuscles  in  A and  B  will  increase  the  tendency  of  the  corpuscles to  leave  their  atoms,  and  if  the  increase  in  the  kin- etic energy  is  considerable  A  and  B  may  each  lose one  or  more  corpuscles.  The  departure  of  a  cor- puscle will  leave  A  and  B  positively  charged, and  they  will  tend  to  separate  under  the  repulsion of  these  charges.  When  separated  they  will  have each  a  positive  charge ;  but  as  there  are  now  free corpuscles  with  negative  charges  moving  about  in the  region  in  which  A  and  B  are  situated,  these positive  charges  will  ultimately  be  neutralized  by
 
 
100  ELECTRICITY    AND  MATTER
 
corpuscles  striking  against  A  and  B  and  remain- ing in  combination  with  them.
 
We  thus  conclude  that  unless  the  corpuscular temperature  after  union  is  less  than  a  certain  limit- ing value,  the  union  cannot  be  permanent,  the complex  formed  being  unstable,  and  incapable  of a  permanent  existence.  Now,  the  corpuscular temperature  of  the  aggregate  formed  by  A  and  B will  depend  upon  the  corpuscular  temperatures  of A  and  B  before  union,  and  also  upon  the  diminu- tion in  the  potential  energy  of  the  system  occa- sioned by  the  union  of  A  and  B.  If  the  corpuscu- lar temperatures  of  A  and  B  before  union  were very  high,  the  corpuscular  temperature  after  union would  be  high  also;  if  they  were  above  a  cer- tain limit,  the  corpuscular  temperature  after  union would  be  too  high  for  stability,  and  the  aggre- gate AB  would  not  be  formed.  Thus,  one  con- dition for  the  formation  of  complex  aggregate's  is that  the  corpuscular  temperature  of  their  constitu- ents before  combination  should  be  sufficiently  low.
 
If  the  molecula/t*  temperature  of  the  gas  in which  A  and  B  are  molecules  is  very  high,  com- bination may  be  prevented  by  the  high  relative velocity  of  A  and  B  carrying  them  apart  in  spite of  their  mutual  attraction.  The  point,  however,
 
 
CONSTITUTION  OF  THE  ATOM  JQ1
 
which  I  wish  to  emphasize  is,  that  we  cannot  se- cure the  union  merely  by  lowering  the  molecular temperature,  i.e.,  by  cooling  the  gas  ;  union  will be  impossible  unless  the  corpuscular  temperature, i.e.,  the  kinetic  energy  due  to  the  motion  of  the corpuscles  inside  the  atom,  is  reduced  below  a  cer- tain value.  We  may  prevent  union  by  raising  the molecular  temperature  of  a  gas,  but  we  cannot  en- sure union  by  lowering  it.
 
Thus,  to  take  a  specific  example,  the  reason,  on this  view,  why  the  atoms  of  hydrogen  present  on the  earth  do  not  combine  to  form  some  other  ele- ment, even  at  the  exceedingly  low  temperature  at which  hydrogen  becomes  liquid,  is  that  even  at this  temperature  the  kinetic  energy  of  the  corpus- cles inside  the  atom,  i.e.,  the  corpuscular  tempera- ture, is  too  great.  It  may  be  useful  to  repeat  here what  we  stated  before,  that  there  is  no  very  inti- mate connection  between  the  corpuscular  and  mo- lecular temperatures,  and  that  we  may  reduce  the latter  almost  to  the  absolute  zero  without  greatly affecting  the  former.
 
We  shall  now  proceed  to  discuss  the  bearing  of these  results  on  the  theory  that  the  different chemical  elements  have  been  gradually  evolved  by the  aggregation  of  primordial  units.
 
 
102  ELECTRICITY    AND    MATTER
 
Let  us  suppose  that  the  first  stage  has  been reached  and  that  we  have  a  number  of  systems formed  by  the  union  of  two  units.  When  first these  binary  systems,  as  we  shall  call  them,  were formed,  the  corpuscles  in  the  system  would  have a  considerable  amount  of  kinetic  energy.  This would  be  so,  because  when  the  two  units  have come  together  there  must  be  an  amount  of  kinetic energy  produced  equal  to  the  diminution  in  the potential  energy  consequent  upon  the  coalescence  of the  two  units.  As  these  binary  systems  have  or- iginally high  corpuscular  temperatures  they  will not  be  likely  to  combine  with  each  other  or  with another  unit ;  before  they  can  do  so  the  kinetic energy  of  the  corpuscles  must  get  reduced.
 
We  shall  proceed  immediately  to  discuss  the way  in  which  this  reduction  is  effected,  but  we shall  anticipate  the  result  of  the  discussion  by saying  that  it  leads  to  the  result  that  the  rate  of decay  in  the  corpuscular  temperature  probably varies  greatly  from  one  binary  system  to  another.
 
Some  of  the  systems  will  therefore  probably have  reached  a  condition  in  which  they  are  able to  combine  with  each  other  or  with  a  single  unit long  before  others  are  able  to  do  so.  The  systems of  the  first  kind  will  combine,  and  thus  we  shall
 
 
CONSTITUTION  OF  THE  ATOM  1Q3
 
have  systems  formed,  some  of  which  contain  three, others  four  units,  while  at  the  same  time  there  are many  of  the  binary  systems  left.  Thus,  the  appear- ance of  the  more  complex  systems  need  not  be simultaneous  with  the  disappearance  of  all  the  sim- pler ones.
 
The  same  principle  will  apply  to  the  formation of  further  aggregations  by  the  systems  containing three  or  four  units ;  some  of  these  will  be  ready  to unite  before  the  others,  and  we  may  have  systems containing  eight  units  formed  before  the  more  per- sistent of  those  containing  four,  three,  two  or  even one  unit  have  disappeared.  With  the  further  ad- vance of  aggregation  the  number  of  different  sys- tems present  at  one  and  the  same  time  will  in- crease.
 
Thus,  if  we  regard  the  systems  containing  differ- ent numbers  of  units  as  corresponding  to  the different  chemical  elements,  then  as  the  universe gets  older  elements  of  higher  and  higher  atomic weight  may  be  expected  to  appear.  Their  appear- ance, however,  will  not  involve  the  annihilation  of the  elements  of  lower  atomic  weight.  The  number of  atoms  of  the  latter  will,  of  course,  diminish, since  the  heavier  elements  are  by  hypothesis  built up  of  material  furnished  by  the  lighter.  The  whole
 
 
104  ELECTRICITY    AND  MATTER
 
of  the  atoms  of  the  latter  would  not,  however,  all be  used  up  at  once,  and  thus  we  may  have  a  very large  number  of  elements  existing  at  one  and  the same  time.
 
If,  however,  there  is  a  continual  fall  in  the  cor- puscular temperature  of  the  atoms  through  radia- tion, the  lighter  elements  will  disappear  in  time, and  unless  there  is  disintegration  of  the  heavier atoms,  the  atomic  weight  of  the  lightest  element surviving  will  continually  increase.  On  this  view, since  hydrogen  is  the  lightest  known  element  and the  atom  of  hydrogen  contains  about  a  thousand corpuscles,  all  aggregations  of  less  than  a  thousand units  have  entered  into  combination  and  are  no longer  free.
 
Tfie  way  the  Corpuscles  in  tlie  Atom  Lose  or  Gain Kinetic  Energy
 
If  the  kinetic  energy  arising  from  the  motion  of the  corpuscles  relatively  to  the  centre  of  gravity of  the  atom  could  by  collisions  be  transformed into  kinetic  energy  due  to  the  motion  of  the  atom as  a  whole,  i.e.,  into  molecular  temperature,  it would  follow  from  the  kinetic  theory  of  gases, since  the  number  of  corpuscles  in  the  atom  is  ex- ceedingly large,  that  the  specific  heat  of  a  gas  at
 
 
CONSTITUTION  OF  THE    ATOM  105
 
constant  pressure  would  be  very  nearly  equal  to the  specific  heat  at  constant  volume ;  whereas,  as a  matter  of  fact,  in  no  gas  is  there  any  approach to  equality  in  these  specific  heats.  We  conclude, therefore,  that  it  is  not  by  collisions  that  the kinetic  energy  of  the  corpuscles  is  diminished.
 
We  have  seen,  however  (page  68),  that  a  mov- ing electrified  particle  radiates  energy  whenever its  velocity  is  changing  either  in  magnitude  or direction.  The  corpuscles  in  the  atom  will  thus emit  electric  waves,  radiating  energy  and  so  losing kinetic  energy.
 
The  rate  at  which  energy  is  lost  in  this  way  by the  corpuscles  varies  very  greatly  with  the  num- ber of  the  corpuscles  and  the  way  in  which  they are  moving.  Thus,  if  we  have  a  single  corpuscle describing  a  circular  orbit  of  radius  a  with  uni- form velocity  v,  the  loss  of  energy  due  to  radia- tion per  second  is  -  =^-5,  where  e  is  the  charge  on 3  V  or
 
the  corpuscles  and  V  the  velocity  of  light.  If instead  of  a  single  corpuscle  we  had  two  corpus- cles at  opposite  ends  of  a  diameter  moving  round the  same  orbit  with  the  same  velocity  as  the  sin- gle corpuscle,  the  loss  of  energy  per  second  from  the two  would  be  very  much  less  than  from  the  single
 
 
106  ELECTRICITY    AND    MATTER
 
corpuscle,  and  the  smaller  the  velocity  of  the  cor- puscle the  greater  would  be  the  diminution  in the  loss  of  energy  produced  by  increasing  the number  of  corpuscles.  The  effect  produced  by increasing  the  number  of  corpuscles  is  shown  in the  following  table,  which  gives  the  rate  of  radia- tion for  each  corpuscle  for  various  numbers  of corpuscles  arranged  at  equal  angular  intervals round  the  circular  orbit.
 
The  table  applies  to  two  cases  ;  in  one  the  veloc- ity of  the  corpuscles  is  taken  as  one-tenth  that  of light,  and  in  the  second  as  one-hundredth.  The radiation  from  a  single  corpuscle  is  in  each  case taken  as  unity.
 
Number  of  corpuscles.        Radiation  from  each  corpuscle.
 
_F  V
 
~~IQ  ~  TOO
 
1 1  1
 
2 9.6  x  10~2  9.6  x  ID"4
 
3 4.6  x  l(r3  4.6  x  10-7
 
4 1.7x10-^  1.7  x  10-10
 
5 5.6X10-5  5.6  x  10~13
 
6 1.6  x  10~7  1.6xlO~17
 
Thus,  we  see  that  the  radiation  from  each  of  a group  of  six  corpuscles  moving  with  one-tenth  the velocity  of  light  is  less  than  one-five-millionth  part of  the  radiation  from  a  single  corpuscle,  describ-
 
 
CONSTITUTION    OF    THE    ATOM  1Q7
 
ing  the  same  orbit  with  the  same  velocity,  while, when  the  velocity  of  the  corpuscles  is  only  one- hundredth  of  that  of  light,  the  reduction  in  the radiation  is  very  much  greater.
 
If  the  corpuscles  are  displaced  from  the  sym- metrical position  in  which  they  are  situated  at equal  intervals  round  a  circle  whose  centre  is  at rest,  the  rate  of  radiation  will  be  very  much  in- creased. In  the  case  of  an  atom  containing  a  large number  of  corpuscles  the  variation  in  the  rate  at which  energy  is  radiated  will  vary  very  rapidly with  the  way  the  corpuscles  are  moving  about  in the  atom.  Thus,  for  example,  if  we  had  a  large number  of  corpuscles  following  close  on  one  an- other's heels  round  a  circular  orbit  the  radiation would  be  exceedingly  small ;  it  would  vanish  alto- gether if  the  corpuscles  were  so  close  together that  they  formed  a  continuous  ring  of  negative electrification.  If  the  same  number  of  particles were  moving  about  irregularly  in  the  atom,  then though  the  kinetic  energy  possessed  by  the  cor- puscles in  the  second  case  might  be  no  greater than  in  the  first,  the  rate  of  radiation,  i.e.,  of  cor- puscular cooling,  would  be  immensely  greater.
 
Thus,  we  see  that  in  the  radiation  of  energy from  corpuscles  whose  velocity  is  not  uniform  we
 
 
10g  ELECTRICITY    AND    MATTER
 
have  a  process  going  on  which  will  gradually  cool the  corpuscular  temperature  of  the  atom,  and  so, if  the  view  we  have  been  discussing  is  correct, enable  the  atom  to  form  further  aggregations  and thus  tend  to  the  formation  of  new  chemical  ele- ments.
 
This  cooling  process  must  be  an  exceedingly slow  one,  for  although  the  corpuscular  tempera- ture when  the  atom  of  a  new  element  is  formed is  likely  to  be  exceedingly  high,  and  the  lowering in  that  temperature  required  before  the  atom  can enter  again  into  fresh  aggregations  very  large, yet  we  have  evidence  that  some  of  the  elements must  have  existed  unchanged  for  many  thousands, nay,  millions  of  years ;  we  have,  indeed,  no  direct evidence  of  any  change  at  all  in  the  atom.  I think,  however,  that  some  of  the  phenomena  of radio-activity  to  which  I  shall  have  to  allude  later, afford,  I  will  not  say  a  proof  of,  but  a  very  strong presumption  in  favor  of  some  such  secular  changes taking  place  in  the  atom.
 
We  must  remember,  too,  that  the  corpuscles  in any  atom  are  receiving  and  absorbing  radiation from  other  atoms.  This  will  tend  to  raise  the corpuscular  temperature  of  the  atom  and  thus help  to  lengthen  the  time  required  for  that
 
 
CONSTITUTION  OF  THE    ATOM  \QQ
 
temperature  to    fall  to    the    point  where  fresh aggregations  of  the  atom  may  be  formed.
 
The  fact  that  the  rate  of  radiation  depends  so much  upon  the  way  the  corpuscles  are  moving about  in  the  atom  indicates  that  the  lives  of  the different  atoms  of  any  particular  element  will  not be  equal ;  some  of  these  atoms  will  be  ready  to enter  upon  fresh  changes  long  before  the  others. It  is  important  to  realize  how  large  are  the amounts  of  energy  involved  in  the  formation  of  a complex  atom  or  in  any  rearrangement  of  the  con- figuration of  the  corpuscles  inside  it.  If  we  have an  atom  containing  n  corpuscles  each  with  a charge  e  measured  in  electrostatic  units,  the  total quantity  of  negative  electricity  in  the  atom  is  n  e and  there  is  an  equal  quantity  of  positive  elec- tricity distributed  through  the  sphere  of  positive electrification;  hence,  the  work  required  to  sep- arate the  atom  into  its  constituent  units  will  be
 
comparable  with  ^ — '- ,  a  being  the  radius  of  the
 
sphere  containing  the  corpuscles.    Thus,  as  the atom  has  been  formed  by  the  aggregation  of  these
 
units  v16'  will  be  of  the  same  order  of  magni- a
 
tude  as  the  kinetic  energy  imparted  to  those  con-
 
 
HO  ELECTRICITY  AND  MATTER
 
stituents  during  their  whole  history,  from  the time  they  started  as  separate  units,  down  to  the time  they  became  members  of  the  atom  under consideration.  They  will  in  this  period  have  radi- ated away  a  large  quantity  of  this  energy,  but  the following  calculation  will  show  what  an  enormous amount  of  kinetic  energy  the  corpuscles  in  the atom  must  possess  even  if  they  have  only  retained an  exceedingly  small  fraction  of  that  communi-
 
cated to  them.    Let  us  calculate  the  value  of  *  —  '-
 
a
 
for  all  the  atoms  in  a  gram  of  the  substance  ;  let N  be  the  number  of  these  atoms  in  a  gram,  then
 
N^-  —  '-  is  the  value  of  the  energy  acquired  by  these atoms.  If  Mis  the  mass  of  an  atom  NM=  1,  thus  :
 
 
but  if  m  is  the  mass  of  a  corpuscle and  therefore
 
 
_
 
a  ma'
 
 
now  when  e  is  measured  in  electrostatic  units
 
<L  =  3  x  1017  and  e  =  3.4  X  1Q-10  ; m
 
 
CONSTITUTION  OF  THE  ATOM  m
 
and  therefore
 
N&*y=  10.2X107X  -.          (1) «  a
 
Let  us  take  the  case  of  the  hydrogen  atom  for which  n  =  1000,  and  take  for  a  the  value  usually assumed  in  the  kinetic  theory  of  gases  for  the radius  of  the  atom,  i.e.,  10"8  cm.  then
 
jy  (M-=  1.02  X  1019ergs;
 
this  amount  of  energy  would  be  sufficient  to  lift  a million  tons  through  a  height  considerably  ex- ceeding one  hundred  yards.  We  see,  too,  from (1)  that  this  energy  is  proportional  to  the  num- ber of  corpuscles,  so  that  the  greater  the  molecu- lar weight  of  an  element,  the  greater  will  be  the amount  of  energy  stored  up  in  the  atoms  in  each gram.
 
We  shall  return  to  the  subject  of  the  internal changes  in  the  atom  when  we  discuss  some  of the  phenomena  of  radio-activity,  but  before  doing so  it  is  desirable  to  consider  more  closely  the  way the  corpuscles  arrange  themselves  in  the  atom. We  shall  begin  with  the  case  where  the  corpuscles are  at  rest.  The  corpuscles  are  supposed  to  be  in a  sphere  of  uniform  positive  electrification  which produces  a  radial  attractive  force  on  each  cor-
 
 
112
 
 
ELECTRICITY    AND    MATTER
 
 
Flo.  16.
 
 
puscle  proportional  to  its  distance  from  the  centre of  the  sphere,  and  the  problem  is  to  arrange  the corpuscles  in  the  sphere  so  that they  are  in  equilibrium  under  this attraction  and  their  mutual  re- pulsions. If  there  are  only  two corpuscles,  A  B,  we  can  see  at once  that  they  will  be  in  equi- librium if  placed  so  that  A  B and  the  centre  of  the  sphere  are  in  the  same straight  line  and  OA  =  OB  =  $  the  radius  of  the sphere.
 
If  there  are  three  corpuscles,  A  B  C,  they  will be  in  equilibrium  of  A  B  C  as  an  equilateral  tri- angle with  its  centre  at  O  and OA=  OB  =  OC  =  (\y,  or  .57 times  the  radius  of  the  sphere.
 
If  there  are  four  corpuscles these  will  be  in  equilibrium  if placed  at  the  angular  points  of  a regular  tetrahedron  with  its  cen- tre at  the  centre  of  the  sphere.  In  these  cases  the corpuscles  are  all  on  the  surface  of  a  sphere  con- centric with  the  sphere  of  positive  electrification, and  we  might  suppose  that  whatever  the  number of  corpuscles  the  position  of  equilibrium  would  be
 
 
FIG.  15.
 
 
CONSTITUTION  OF  THE    ATOM  H3
 
one  of  symmetrical  distribution  over  the  surface of  a  sphere.  Such  a  distribution  would  indeed technically  be  one  of  equilibrium,  but  a  mathe- matical calculation  shows  that  unless  the  number of  corpuscles  is  quite  small,  say  seven  or  eight  at the  most,  this  arrangement  is  unstable  and  so  can never  persist.  When  the  number  of  corpuscles  is greater  than  this  limiting  number,  the  corpuscles break  up  into  two  groups.  One  group  containing the  smaller  number  of  corpuscles  is  on  the  surface of  a  small  body  concentric  with  the  sphere ;  the remainder  are  on  the  surface  of  a  larger  concen- tric body.  When  the  number  of  corpuscles  is still  further  increased  there  comes  a  stage  when the  equilibrium  cannot  be  stable  even  with  two groups,  and  the  corpuscles  now  divide  themselves into  three  groups,  arranged  on  the  surfaces  of  con- centric shells ;  and  as  we  go  on  increasing  the number  we  pass  through  stages  in  which  more  and more  groups  are  necessary  for  equilibrium.  With any  considerable  number  of  corpuscles  the  prob- lem of  finding  the  distribution  when  in  equilibrium becomes  too  complex  for  calculation  ;  and  we  have to  turn  to  experiment  and  see  if  we  can  make  a model  in  which  the  forces  producing  equilibrium are  similar  to  those  we  have  supposed  to  be  at
 
 
114  ELECTRICITY    AND    MATTER
 
work  in  the  corpuscle.  Such  a  model  is  afforded by  a  very  simple  and  beautiful  experiment  first made,  I  think,  by  Professor  Mayer.  In  this  experi- ment a  number  of  little  magnets  are  floated  in  a vessel  of  water.  The  magnets  are  steel  needles magnetized  to  equal  strengths  and  are  floated  by being  thrust  through  small  disks  of  cork.  The magnets  are  placed  so  that  the  positive  poles  are either  all  above  or  all  below  the  surface  of  the water.  These  positive  poles,  like  the  corpuscles, repel  each  other  with  forces  varying  inversely  as the  distance  between  them.  The  attractive  force is  provided  by  a  negative  pole  (if  the  little  mag- nets have  their  positive  poles  above  the  water)  sus- pended some  distance  above  the  surface  of  the water.  This  pole  will  exert  on  the  positive  poles of  the  little  floating  magnets  an  attractive  force the  component  of  which,  parallel  to  the  surface of  the  water,  will  be  radial,  directed  to  0,  the projection  of  the  negative  pole  on  the  surface  of the  water,  and  if  the  negative  pole  is  some  dis- tance above  the  surface  the  component  of  the  force to  O  will  be  very  approximately  proportional  to the  distance  from  O.  Thus  the  forces  on  the  poles of  the  floating  magnets  will  be  very  similar  to  those acting  on  the  corpuscle  in  our  hypothetical  atom ;
 
 
CONSTITUTION  OF  THE    ATOM
 
 
H5
 
 
the  chief  difference  being  that  the  corpuscles  are free  to  move  about  in  all  directions  in  space,  while the  poles  of  the  floating  magnets  are  constrained to  move  in  a  plane  parallel  to  the  surface  of  the water.
 
The  configurations  which  the  floating  magnets assume  as  the  number  of  magnets  increases  from two  up  to  nineteen  is  shown  in  Fig.  17,  which was  given  by  Mayer.
 
 
FIG.  17.
 
The  configuration  taken  up  when  the  magnets are  more  numerous  can  be  found  from  the  follow- ing table,  which  is  also  due  to  Mayer.  From  this table  it  will  be  seen  that  when  the  number  of floating  magnets  does  not  exceed  five  the  magnets
 
 
116
 
 
ELECTRICITY    AND  MATTER
 
 
arrange  themselves  at  the  corners  of  a  regular polygon,  five  at  the  corners  of  a  pentagon,  four  at the  corners  of  a  square  and  so  on.  When  the number  is  greater  than  five  this  arrangement  no longer  holds.  Thus,  six  magnets  do  not  arrange themselves  at  the  corners  of  a  hexagon,  but  divide into  two  systems,  one  magnet  being  at  the  centre and  five  outside  it  at  the  corners  of  a  regular  penta- gon. This  arrangement  in  two  groups  lasts  until there  are  fifteen  magnets,  when  we  have  three groups ;  with  twenty-seven  magnets  we  get  four groups  and  so  on.
 
 
Arrangement  of  Magi
 
tets  (Mayer)
 
1.                  2.                    3.
 
4.                    6.
 
1-5        (2.6            (3.7 •1-6          (2.7              (3.8
 
I4-8  I5-9
 
(4.9      (
 
1  .  7
 
1.6.9    C2  .  7  .  10        3.7      10
 
(4  .  8  .  12    J5  .  9  .    12
 
1.6.9    J  2  .  8  .  10
 
3  .  7  .  11
 
1  4  .  8  .  13    1  5  .  9  .  13
 
1  .  6  .  10  (2  .  7  .  11 1  .  6  .  11
 
3  .  8  .  10 3  .  8  .  11
 
»4  .  9  .  12 4  .  9  .  13
 
3  .  8  .  12
 
3  .  8  .  13
 
CONSTITUTION  OF  THE    ATOM
 
 
f  •  6
 
.  5
 
.  9  . .  9  .
 
12  (  2  .  7  .  10  .  15
 
13  \2  .  7  .  12  .  14
 
3  .  7  .  12  .  13  f4  .  9  .  13  .  14 3  .  7  .  12  .  14  J  4  .9  .  13  .  15
 
.  6
 
.  9  .
 
12
 
3.7.  13  .  14  (4  .  9  .  14  .  15
 
.  6
 
.  10  .
 
12
 
3.7  13  .  15
 
.  C
 
.  10  .
 
13
 
.  6
 
.  11  .
 
2
 
.  6
 
.  11  .
 
13
 
.  6
 
.  11  .
 
14
 
.  •  6
 
.  11  .
 
15
 
Where,  for  example,  3.  7.  12.  13  means  that thirty-five  magnets  arrange  themselves  so  that there  is  a  ring  of  three  magnets  inside,  then  a  ring of  seven,  then  one  of  twelve,  and  one  of  thirteen outside.
 
I  think  this  table  affords  many  suggestions  tow- ard the  explanation  of  some  of  the  properties possessed  by  atoms.  Let  us  take,  for  example,  the chemical  law  called  the  Periodic  Law ;  according to  this  law  if  we  arrange  the  elements  in  order  of increasing  atomic  weights,  then  taking  an  element of  low  atomic  weight,  say  lithium,  we  find  certain properties  associated  with  it.  These  properties are  not  possessed  by  the  elements  immediately following  it  in  the  series  of  increasing  atomic weight ;  but  they  appear  again  when  we  come  to sodium,  then  they  disappear  again  for  a  time,
 
 
Ug  ELECTRICITY    AND  MATTER
 
but  reappear  when  we  reach  potassium,  and  so on.  Let  us  now  consider  the  arrangements  of the  floating  magnets,  and  suppose  that  the  number of  magnets  is  proportional  to  the  combining  weight of  an  element.  Then,  if  any  property  were  asso- ciated with  the  triangular  arrangement  of  magnets, it  would  be  possessed  by  the  elements  whose  com- bining weight  was  on  this  scale  three,  but  would not  appear  again  until  we  reached  the  combining weight  ten,  when  it  reappears,  as  for  ten  magnets we  have  the  triangular  arrangement  in  the  middle and  a  ring  of  seven  magnets  outside.  When  the number  of  magnets  is  increased  the  triangular arrangement  disappears  for  a  time,  but  reappears with  twenty  magnets,  and  again  with  thirty-five, the  triangular  arrangement  appearing  and  dis- appearing in  a  way  analogous  to  the  behavior  of the  properties  of  the  elements  in  the  Periodic Law.  As  an  example  of  a  property  that  might very  well  be  associated  with  a  particular  grouping of  the  corpuscles,  let  us  take  the  times  of  vibra- tion of  the  system,  as  shown  by  the  position  of the  lines  in  the  spectrum  of  the  element.  First let  us  take  the  case  of  three  corpuscles  by  them- selves in  the  positively  electrified  sphere.  The three  corpuscles  have  nine  degrees  of  freedom,  so
 
 
CONSTITUTION  OF  THE  ATOM  HQ
 
that  there  are  nine  possible  periods.  Some  of these  periods  in  this  case  would  be  infinitely  long, and  several  of  the  possible  periods  would  be  equal to  each  other,  so  that  we  should  not  get  nine  dif- ferent periods.
 
Suppose  that  the  lines  in  the  spectrum  of  the three  corpuscles  are  as  represented  in  Fig.  18  a,
 
 
A
 
 
a  c  D    c
 
 
e  3    /    2
 
 
A    B    c    a    £
 
 
FIG.  18.
 
where  the  figures  under  the  lines  represent  the number  of  periods  which  coalesce  at  that  line ;  i.e., regarding  the  periods  as  given  by  an  equation  with nine  roots,  we  suppose  that  there  is  only  one  root giving  the  period  corresponding  to  the  line  -4, while  corresponding  to  I>  there  are  two  equal roots,  three  equal  roots  corresponding  to  <7,  one
 
 
120  ELECTRICITY    AND    MATTER
 
root,  to  O,  and  two  to  E.  These  periods  would have  certain  numerical  relations  to  each  other,  in- dependent of  the  charge  on  the  corpuscle,  the  size of  the  sphere  in  which  they  are  placed,  or  their distance  from  the  centre  of  the  sphere.  Each  of these  quantities,  although  it  does  not  affect  the ratio  of  the  periods,  will  have  a  great  effect  upon the  absolute  value  of  any  one  of  them.  Now, suppose  that  these  three  corpuscles,  instead  of being  alone  in  the  sphere,  form  but  one  out  of several  groups  in  it,  just  as  the  triangle  of  mag- nets forms  a  constituent  of  the  grouping  of  3,  10, 20,  and  35  magnets.  Let  us  consider  how  the presence  of  the  other  groups  would  affect  the periods  of  vibration  of  the  three  corpuscles.  The absolute  values  of  the  periods  would  generally  be entirely  different,  but  the  relationship  existing  be- tween the  various  periods  would  be  much  more persistent,  and  although  it  might  be  modified  it would  not  be  destroyed.  Using  the  phraseology  of the  Planetary  Theory,  we  may  regard  the  motion of  the  three  corpuscles  as  "  disturbed  "  by  the other  groups.
 
When  the  group  of  three  corpuscles  was  by  it- self there  were  several  displacements  which  gave the  same  period  of  vibration ;  for  example,  corre-
 
 
CONSTITUTION  OF  THE  ATOM  121
 
spending  to  the  line  C  there  were  three  displace- ments, all  giving  the  same  period.  When,  how^ ever,  there  are  other  groups  present,  then  these different  displacements  will  no  longer  be  sym- metrical with  respect  to  these  groups,  so  that  the three  periods  will  no  longer  be  quite  equal.  They would,  however,  be  very  nearly  equal  unless  the effect  of  the  other  groups  is  very  large.  Thus, in  the  spectrum,  <7,  instead  of  being  a  single  line, would  become  a  triplet,  while  B  and  E  would  be- come doublets.  A  D  would  remain  single  lines.
 
Thus,  the  spectrum  would  now  resemble  Fig. 18  b',  the  more  groups  there  are  surrounding  the group  of  three  the  more  will  the  motion  of  the latter  be  disturbed  and  the  greater  the  separation of  the  constituents  of  the  triplets  and  doublets. The  appearance  as  the  number  of  groups  increases is  shown  in  Fig.  18  £,  c.  Thus,  if  we  regarded the  element  which  contain  this  particular  group- ing of  corpuscles  as  being  in  the  same  group  in  the classification  of  elements  according  to  the  Periodic Law,  we  should  get  in  the  spectra  of  these  ele- ments homologous  series  of  lines,  the  distances  be- tween the  components  of  the  doublets  and  triplets increasing  with  the  atomic  weight  of  the  elements. The  investigations  of  Rydberg,  Runge  and  Pas-
 
 
122  ELECTRICITY    AXD    MATTER
 
chen  and  Keyser  have  shown  the  existence  in  the spectra  of  elements  of  the  same  group  series  of lines  having  properties  in  many  respects  analogous to  those  we  have  described.
 
Another  point  of  interest  given  by  Mayer's  ex- periments is  that  there  is  more  than  one  stable configuration  for  the  same  number  of  magnets; these  configurations  correspond  to  different  amounts of  potential  energy,  so  that  the  passage  from  the configuration  of  greater  potential  energy  to  that  of less  would  give  kinetic  energy  to  the  corpuscle. From  the  values  of  the  potential  energy  stored in  the  atom,  of  which  we  gave  an  estimate  on page  111,  we  infer  that  a  change  by  even  a  small fraction  in  that  potential  energy  would  develop an  amount  of  kinetic  energy  which  if  converted into  heat  would  greatly  transcend  the  amount  of heat  developed  when  the  atoms  undergo  any  known chemical  combination.
 
An  inspection  of  the  table  shows  that  there  are certain  places  in  it  where  the  nature  of  the  con- figuration changes  very  rapidly  with  the  number of  magnets  ;  thus,  five  magnets  form  one  group, while  six  magnets  form  two;  fourteen  magnets form  two  groups,  fifteen  three ;  twenty  -  seven magnets  form  three  groups,  twenty-eight  four,
 
 
CONSTITUTION  OF  THE  ATOM  123
 
and  so  on.  If  we  arrange  the  chemical  elements in  the  order  of  their  atomic  weights  we  find  there are  certain  places  where  the  difference  in  proper- ties of  consecutive  elements  is  exceptionally  great ; thus,  for  example,  we  have  extreme  differences  in properties  between  fluorine  and  sodium.  Then there  is  more  or  less  continuity  in  the  properties until  we  get  to  chlorine,  which  is  followed  by potassium;  the  next  break  occurs  at  bromine and  rubidium  and  so  on.  This  effect  seems analogous  to  that  due  to  the  regrouping  of  the magnets.
 
So  far  we  have  supposed  the  corpuscles  to  be at  rest ;  if,  however,  they  are  in  a  state  of  steady motion  and  describing  circular  orbits  round  the centre  of  the  sphere,  the  effect  of  the  centrifugal force  arising  from  this  motion  will  be  to  drive  the corpuscles  farther  away  from  the  centre  of  the sphere,  without,  in  many  cases,  destroying  the character  of  the  configuration.  Thus,  for  example, if  we  have  three  corpuscles  in  the  sphere,  they will,  in  the  state  of  steady  motion,  as  when  they are  at  rest,  be  situated  at  the  corners  of  an  equi- angular triangle ;  this  triangle  will,  however,  be rotating  round  the  centre  of  the  sphere,  and  the distance  of  the  corpuscles  from  the  centre  will  be
 
 
124  ELECTRICITY    AND    MATTER
 
greater  than  when  they  are  at  rest  and  will  in- crease with  the  velocity  of  the  corpuscles.
 
There  are,  however,  many  cases  in  which  rota- tion is  essential  for  the  stability  of  the  configura- tion. Thus,  take  the  case  of  four  corpuscles. These,  if  rotating  rapidly,  are  in  stable  steady motion  when  at  the  corners  of  a  square,  the  plane of  the  square  being  at  right  angles  to  the  axis  of rotation ;  when,  however,  the  velocity  of  rotation of  the  corpuscles  falls  below  a  certain  value,  the arrangement  of  four  corpuscles  in  one  plane  be- comes unstable,  and  the  corpuscles  tend  to  place themselves  at  the  corners  of  a  regular  tetrahedron, which  is  the  stable  arrangement  when  the  cor- puscles are  at  rest.  The  system  of  four  corpuscles at  the  corners  of  a  square  may  be  compared  with a  spinning  top,  the  top  like  the  corpuscles  being unstable  unless  its  velocity  of  rotation  exceeds a  certain  critical  value.  Let  us  suppose  that initially  the  velocity  of  the  corpuscles  exceeds this  value,  but  that  in  some  way  or  another  the corpuscles  gradually  lose  their  kinetic  energy; the  square  arrangement  will  persist  until  the  ve- locity of  the  corpuscles  is  reduced  to  the  critical value.  The  arrangement  will  then  become  un- stable, and  there  will  be  a  convulsion  in  the  sys-
 
 
CONSTITUTION    OF  THE  ATOM  125
 
tern  accompanied  by  a  great  evolution  of  kinetic energy.
 
Similar  considerations  will  apply  to  many  as- semblages of  corpuscles.  In  such  cases  the  con- figuration when  the  corpuscles  are  rotating  with great  rapidity  will  (as  in  the  case  of  the  four  cor- puscles) be  essentially  different  from  the  configu- ration of  the  same  number  of  corpuscles  when  at rest.  Hence  there  must  be  some  critical  velocity of  the  corpuscles,  such  that,  for  velocities  greater than  the  critical  one,  a  configuration  is  stable, which  becomes  unstable  when  the  velocity  is reduced  below  the  critical  value.  When  the  ve- locity sinks  below  the  critical  value,  instability sets  in,  and  there  is  a  kind  of  convulsion  or  ex- plosion, accompanied  by  a  great  diminution  in  the potential  energy  and  a  corresponding  increase  in the  kinetic  energy  of  the  corpuscles.  This  increase in  the  kinetic  energy  of  the  corpuscles  may  be sufficient  to  detach  considerable  numbers  of  them from  the  original  assemblage.
 
These  considerations  have  a  very  direct  bearing on  the  view  of  the  constitution  of  the  atoms  which we  have  taken  in  this  chapter,  for  they  show  that with  atoms  of  a  special  kind,  i.e.,  with  special atomic  weights,  the  corpuscular  cooling  caused  by
 
 
126  ELECTRICITY    AND    MATTER
 
the  radiation  from  the  moving  corpuscles  which we  have  supposed  to  be  slowly  going  on,  might, when  it  reached  a  certain  stage,  produce  instabil- ity inside  the  atom,  and  produce  such  an  in- crease in  the  kinetic  energy  of  the  corpuscles  as to  give  rise  to  greatly  increased  radiation,  and  it might  be  detachment  of  a  portion  of  the  atom. It  would  cause  the  atom  to  emit  energy ;  this energy  being  derived  from  the  potential  energy due  to  the  arrangement  of  the  corpuscles  in  the atom.  We  shall  see  when  we  consider  the  phe- nomenon of  radio-activity  that  there  is  a  class  of bodies  which  show  phenomena  analogous  to  those just  described.
 
On  the  view  that  the  lighter  elements  are formed  first  by  the  aggregation  of  the  unit doublet,  the  negative  element  of  which  is  the  cor- puscle, and  that  it  is  by  the  combination  of  the atoms  of  the  lighter  elements  that  the  atoms  of the  heavier  elements  are  produced,  we  should  ex- pect the  corpuscles  in  the  heavy  atoms  to  be  ar- ranged as  it  were  in  bundles,  the  arrangement  of the  corpuscles  in  each  bundle  being  similar  to  the arrangement  in  the  atom  of  some  lighter  element. In  the  heavier  atom  these  bundles  would  act  as subsidiary  units,  each  bundle  corresponding  to
 
 
CONSTITUTION  OF  THE    ATOM  J27
 
one  of  the  magnets  in  the  model  formed  by  the floating  magnets,  while  inside  the  bundle  them- selves the  corpuscle  would  be  the  analogue  of the  magnet.
 
We  must  now  go  on  to  see  whether  an  atom built  up  in  the  way  we  have  supposed  could  pos- sess any  of  the  properties  of  the  real  atom.  Is there,  for  example,  in  this  model  of  an  atom  any scope  for  the  electro-chemical  properties  of  the real  atom ;  such  properties,  for  example,  as  those illustrated  by  the  division  of  the  chemical  ele- ments into  two  classes,  electro-positive  and  electro- negative. Why,  for  example,  if  this  is  the  con- stitution of  the  atom,  does  an  atom  of  sodium  or potassium  tend  to  acquire  a  positive,  the  atom  of chlorine  a  negative  charge  of  electricity  ?  Again, is  there  anything  in  the  model  of  the  atom  to suggest  the  possession  of  such  a  property  as  that called  by  the  chemists  valency  ;  i.e.,  the  property which  enables  us  to  divide  the  elements  into groups,  called  monads,  dyads,  triads,  such  that  in a  compound  formed  by  any  two  elements  of  the first  group  the  molecule  of  the  compound  will contain  the  same  number  of  atoms  of  each  element, while  in  a  compound  formed  by  an  element  A  in the  first  group  with  one  B  in  the  second,  the  mole-
 
 
12g  ELECTRICITY    AND    MATTER
 
cule  of  the  compound  contains  twice  as  many atoms  of  A  as  of  B,  and  so  on  ?
 
Let  us  now  turn  to  the  properties  of  the  model atom.  It  contains  a  very  large  number  of  corpus- cles in  rapid  motion.  We  have  evidence  from  the phenomena  connected  with  the  conduction  of electricity  through  gases  that  one  or  more  of  these corpuscles  can  be  detached  from  the  atom. These  may  escape  owing  to  their  high  veloc- ity enabling  them  to  travel  beyond  the  attrac- tion of  the  atom.  They  may  be  detached  also  by collision  of  the  atom  with  other  rapidly  moving atoms  or  free  corpuscles.  When  once  a  corpuscle has  escaped  from  an  atom  the  latter  will  have  a  pos- itive charge.  This  will  make  it  more  difficult  for a  second  negatively  electrified  corpuscle  to  escape, for  in  consequence  of  the  positive  charge  on  the atom  the  latter  will  attract  the  second  corpuscle more  strongly  than  it  did  the  first.  Now  we  can readily  conceive  that  the  ease  with  which  a  par- ticle will  escape  from,  or  be  knocked  out  of,  an atom  may  vary  very  much  in  the  atoms  of  the  dif- ferent elements.  In  some  atoms  the  velocities  of the  corpuscles  may  be  so  great  that  a  corpuscle escapes  at  once  from  the  atom.  It  may  even  be that  after  one  has  escaped,  the  attraction  of  the
 
 
CONSTITUTION    OF    THE    ATOM  129
 
positive  electrification  thus  left  on  the  atom  is not  sufficient  to  restrain  a  second,  or  even  a  third, corpuscle  from  escaping.  Such  atoms  would  ac- quire positive  charges  of  one,  two,  or  three  units, according  as  they  lost  one,  two,  or  three  corpus- cles. On  the  other  hand,  there  may  be  atoms  in which  the  velocities  of  the  corpuscles  are  so  small that  few,  if  any,  corpuscles  escape  of  their  own accord,  nay,  they  may  even  be  able  to  receive one  or  even  more  than  one  corpuscle  before  the repulsion  exerted  by  the  negative  electrification on  these  foreign  corpuscles  forces  any  of  the original  corpuscles  out.  Atoms  of  this  kind  if placed  in  a  region  where  corpuscles  were  present would  by  aggregation  with  these  corpuscles  re. ceive  a  negative  charge.  The  magnitude  of  the negative  charge  would  depend  upon  the  firmness with  which  the  atom  held  its  corpuscles.  If  a negative  charge  of  one  corpuscle  were  not  suf- ficient to  expel  a  corpuscle  while  the  negative charge  of  two  corpuscles  could  do  so,  the  maxi- mum negative  charge  on  the  atom  would  be  one unit.  If  two  corpuscles  were  not  sufficient  to  expel a  corpuscle,  but  three  were,  the  maximum  nega- tive charge  would  be  two  units,  and  so  on.  Thus, the  atoms  of  this  class  tend  to  get  charged  with
 
 
L30  ELECTRICITY    AND  MATTER
 
negative  electricity  and  correspond  to  the  electro- negative chemical  elements,  while  the  atoms  of  the class  we  first  considered,  and  which  readily  lose corpuscles,  acquire  a  positive  charge  and  corre- spond to  the  atoms  of  the  electro-positive  elements. We  might  conceive  atoms  in  which  the  equilib- rium of  the  corpuscles  was  so  nicely  balanced that  though  they  do  not  of  themselves  lose  a  cor- puscle, and  so  do  not  acquire  a  positive  charge,  the repulsion  exerted  by  a  foreign  corpuscle  coming on  to  the  atom  would  be  sufficient  to  drive  out  a corpuscle.  Such  an  atom  would  be  incapable  of receiving  a  charge  either  of  positive  or  negative electricity.
 
Suppose  we  have  a  number  of  the  atoms  that readily  lose  their  corpuscles  mixed  with  a  num- ber of  those  that  can  retain  a  foreign  corpuscle. Let  us  call  an  atom  of  the  first  class  A,  one  of  the second  £,  and  suppose  that  the  A  atoms  are  of the  kind  that  lose  one  corpuscle  while  the  IB  atoms are  of  the  kind  that  can  retain  one,  but  not  more than  one  ;  then  the  corpuscles  which  escape  from the  A  atoms  will  ultimately  find  a  home  on  the  B atoms,  and  if  there  are  an  equal  number  of  the two  kinds  of  atoms  present  we  shall  get  ultimate- ly all  the  A  atoms  with  the  unit  positive  charge,
 
 
CONSTITUTION  OF  THE  ATOM  131
 
all  the  B  atoms  with  the  unit  negative  charge. These  oppositely  electrified  atoms  will  attract  each other,  and  we  shall  get  the  compound  A  B formed.  If  the  A  atoms  had  been  of  the  kind that  lost  two  corpuscles,  and  the  B  atoms  the same  as  before,  then  the  A  atoms  would  get  the charge  of  two  positive  units,  the  B  atoms  a  charge of  one  unit  of  negative  electricity.  Thus,  to  form a  neutral  system  two  of  the  B  atoms  must  com- bine  with  one  of  the  A's  and  thus  the  compound A  HI  would  be  formed.
 
Thus,  from  this  point  of  view  a  univalent  elec- tro-positive atom  is  one  which,  under  the  circum- stances prevailing  when  combination  is  taking place,  has  to  lose  one  and  only  one  corpuscle  be- fore stability  is  attained ;  a  univalent  electro-neg- ative atom  is  one  which  can  receive  one  but  not more  than  one  corpuscle  without  driving  off  other corpuscles  from  the  atom;  a  divalent  electro- positive atom  is  one  that  loses  two  corpuscles  and no  more,  and  so  on.  The  valency  of  the  atom thus  depends  upon  the  ease  with  which  corpus- cles can  escape  from  or  be  received  by  the  atom ; this  may  be  influenced  by  the  circumstances existing  when  combination  is  taking  place.  Thus, it  would  be  easier  for  a  corpuscle,  when  once  it
 
 
132  ELECTRICITY  AND  MATTER
 
had  got  outside  the  atom,  to  escape  being  pulled back  again  into  it  by  the  attraction  of  its  positive electrification,  if  the  atom  were  surrounded  by  good conductors  than  if  it  were  isolated  in  space.  We can  understand,  then,  why  the  valency  of  an  atom may  in  some  degree  be  influenced  by  the  physical conditions  under  which  combination  is  taking  place. On  the  view  that  the  attraction  between  the atoms  in  a  chemical  compound  is  electrical  in  its origin,  the  ability  of  an  element  to  enter  into chemical  combination  depends  upon  its  atom  hav- ing the  power  of  acquiring  a  charge  of  electricity. This,  on  the  preceding  view,  implies  either  that  the uncharged  atom  is  unstable  and  has  to  lose  one  or more  corpuscles  before  it  can  get  into  a  steady state,  or  else  that  it  is  so  stable  that  it  can  retain one  or  more  additional  corpuscles  without  any  of the  original  corpuscles  being  driven  out.  If  the range  of  stability  is  such  that  the  atom,  though stable  when  uncharged,  becomes  unstable  when  it receives  an  additional  corpuscle,  the  atom  will  not be  able  to  receive  a  charge  either  of  positive  or negative  electricity,  and  will  therefore  not  be  able to  enter  into  chemical  combination.  Such  an  atom would  have  the  properties  of  the  atoms  of  such elements  as  argon  or  helium.
 
 
CONSTITUTION  OF  THE  ATOM  133
 
The  view  that  the  forces  which  bind  together the  atoms  in  the  molecules  of  chemical  compounds are  electrical  in  their  origin,  was  first  proposed by  Berzelius ;  it  was  also  the  view  of  Davy  and  of Faraday.  Helmholtz,  too,  declared  that  the mightiest  of  the  chemical  forces  are  electrical  in their  origin.  Chemists  in  general  seem,  however, to  have  made  but  little  use  of  this  idea,  having apparently  found  the  conception  of  "bonds  of affinity"  more  fruitful.  This  doctrine  of  bonds is,  however,  when  regarded  in  one  aspect  almost identical  with  the  electrical  theory.  The  theory of  bonds  when  represented  graphically  supposes that  from  each  univalent  atom  a  straight  line (the  symbol  of  a  bond)  proceeds;  a  divalent atom  is  at  the  end  of  two  such  lines,  a  trivalent atom  at  the  end  of  three,  and  so  on ;  and  that when  the  chemical  compound  is  represented  by  a graphic  formula  in  this  way,  each  atom  must  be at  the  end  of  the  proper  number  of  the  lines which  represent  the  bonds.  Now,  on  the  electrical view  of  chemical  combination,  a  univalent  atom has  one  unit  charge,  if  we  take  as  our  unit  of charge  the  charge  on  the  corpuscle ;  the  atom  is therefore  the  beginning  or  end  of  one  unit  Fara- day tube :  the  beginning  if  the  charge  on  the
 
 
134  ELECTRICITY    AND    MATTEB
 
atom  is  positive,  the  end  if  the  charge  is  nega- tive. A  divalent  atom  has  two  units  of  charge  and therefore  it  is  the  origin  or  termination  of  two unit  Faraday  tubes.  Thus,  if  we  interpret  the "bond"  of  the  chemist  as  indicating  a  unit  Fara- day tube,  connecting  charged  atoms  in  the  mole- cule, the  structural  formulae  of  the  chemist  can be  at  once  translated  into  the  electrical  theory. There  is,  however,  one  point  of  difference  which deserves  a  little  consideration :  the  symbol  indi- cating a  bond  on  the  chemical  theory  is  not  re- garded as  having  direction  ;  no  difference  is  made on  this  theory  between  one  end  of  a  bond  and the  other.  On  the  electrical  theory,  however,  there is  a  difference  between  the  ends,  as  one  end  cor- responds to  a  positive,  the  other  to  a  negative charge.  An  example  or  two  may  perhaps  be  the easiest  way  of  indicating  the  effect  of  this  consid- eration. Let  us  take  the  gas  ethane  whose  structu- ral formula  is  written
 
 
According  to  the  chemical  view  there  is  no  differ-
 
 
CONSTITUTION    OF  THE    ATOM  135
 
ence  between  the  two  carbon  atoms  in  this  com- pound ;  there  would,  however,  be  a  difference  on the  electrical  view.  For  let  us  suppose  that  the hydrogen  atoms  are  all  negatively  electrified;  the three  Faraday  tubes  going  from  the  hydrogen  atoms to  each  carbon  atom  give  a  positive  charge  of three  units  on  each  carbon  atom.  But  in  addition to  the  Faraday  tubes  coming  from  the  hydrogen atoms,  there  is  one  tube  which  goes  from  one  car- bon atom  to  the  other.  This  means  an  additional positive  charge  on  one  carbon  atom  and  a  nega- tive charge  on  the  other.  Thus,  one  of  the  carbon atoms  will  have  a  charge  of  four  positive  units, while  the  other  will  have  a  charge  of  three  positive and  one  negative  unit,  i.e.,  two  positive  units ;  so that  on  this  view  the  two  carbon  atoms  are  not  in the  same  state.  A  still  greater  difference  must exist  between  the  atoms  when  we  have  what  is called  double  linking,  i.e.,  when  the  carbon  atoms are  supposed  to  be  connected  by  two  bonds,  as  in the  compound
 
 
136  ELECTRICITY    AND    MATT  K II
 
Here,  if  one  carbon  atom  had  a  charge  of  four  posi- tive units,  the  other  would  have  a  charge  of  two positive  and  two  negative  units.
 
We  might  expect  to  discover  such  differences as  are  indicated  by  these  considerations  by  the  in- vestigation of  which  are  known  as  additive  prop- erties, i.e,  properties  which  can  be  calculated when  the  chemical  constitution  of  the  molecule is  known.  Thus,  let  A  B  C  represent  the  atoms  of three  chemical  elements,  then  if  p  is  the  value  of some  physical  constant  for  the  molecule  of  .  I ,, q  the  value  for  13 %,  and  r  for  6'2,  then  if  this  con- stant obeys  the  additive  law,  its  value  for  a  mole- cule of  the  substance  whose  chemical  composition is  represented  by  the  formula  A&  J2y  Cz  is
 
\px-\r\qy-\-\rz.
 
We  can  only  expect  relations  like  this  to  hold  when the  atoms  which  occur  in  the  different  compounds corresponding  to  different  values  of  x  y  z  are the  same.  If  the  atom  A  occurs  in  different  states in  different  compounds  we  should  have  to  use different  values  of  p  for  these  compounds.
 
A  well-known  instance  of  the  additive  prop- erty is  the  refractive  power  of  different  substances for  light,  and  in  this  case  chemists  find  it  neces-
 
 
CONSTITUTION    OP    THE    ATOM  137
 
sary  to  use  different  values  for  the  refraction  due a  carbon  atom  according  as  the  atom  is  doubly  or singly  linked.  They  use,  however,  the  same  value for  the  refraction  of  the  carbon  atom  when  singly linked  with  another  atom  as  when,  as  in  the  com- pound G  HI,  it  is  not  linked  with  another  carbon atom  at  all.
 
It  may  be  urged  that  although  we  can  conceive that  one  atom  in  a  compound  should  be  positively and  the  other  negatively  electrified  when  the atoms  are  of  different  kinds,  it  is  not  easy  to  do so  when  the  atoms  are  of  the  same  kind,  as  they are  in  the  molecules  of  the  elementary  gases HI,  OK  JVj  and  so  on.  With  reference  to  this point  we  may  remark  that  the  electrical  state  of an  atom,  depending  as  it  does  on  the  power  of the  atom  to  emit  or  retain  corpuscles,  may  be  very largely  influenced  by  circumstances  external  to the  atom.  Thus,  for  an  example,  an  atom  in  a  gas when  surrounded  by  rapidly  moving  atoms  or corpuscles  which  keep  striking  against  it  may have  corpuscles  driven  out  of  it  by  these  collisions and  thus  become  positively  electrified.  On  the other  hand,  we  should  expect  that,  ceteris  paribus, the  atom  would  be  less  likely  to  lose  a  corpuscle when  it  is  in  a  gas  than  when  in  a  solid  or  a
 
 
138  ELECTRICITY  AND    MATTER
 
liquid.  For  when  in  a  gas  after  a  corpuscle  has just  left  the  atom  it  has  nothing  beyond  its  own velocity  to  rely  upon  to  escape  from  the  attraction of  the  positively  electrified  atom,  since  the  other atoms  are  too  far  away  to  exert  any  forces  upon it.  When,  however,  the  atom  is  in  a  liquid  or  a solid,  the  attractions  of  the  other  atoms  which crowd  round  this  atom  may,  when  once  a  corpus- cle has  left  its  atom,  help  it  to  avoid  falling back  again  into  atom.  As  an  instance  of  this effect  we  may  take  the  case  of  mercury  in  the liquid  and  gaseous  states.  In  the  liquid  state mercury  is  a  good  conductor  of  electricity.  One way  of  regarding  this  electrical  conductivity  is to  suppose  that  corpuscles  leave  the  atoms  of  the mercury  and  wander  about  through  the  inter- stices between  the  atoms.  These  charged  cor- puscles when  acted  upon  by  an  electric  force are  set  in  motion  and  constitute  an  electric  cur- rent, the  conductivity  of  the  liquid  mercury  in- dicating the  presence  of  a  large  number  of  cor- puscles. When,  however,  mercury  is  in  the  gaseous state,  its  electrical  conductivity  has  been  shown  by Strutt  to  be  an  exceedingly  small  fraction  of  the conductivity  possessed  by  the  same  number  of molecules  when  gaseous.  We  have  thus  indications
 
 
CONSTITUTION    OF  THE    ATOM  139
 
that  the  atoms  even  of  an  electro-positive  sub- stance like  mercury  may  only  lose  comparatively few  corpuscles  when  in  the  gaseous  state.  Sup- pose then  that  we  had  a  great  number  of  atoms all  of  one  kind  in  the  gaseous  state  and  thus  mov- ing about  and  coming  into  collision  with  each other;  the  more  rapidly  moving  ones,  since  they would  make  the  most  violent  collisions,  would  be more  likely  to  lose  corpuscles  than  the  slower ones.  The  faster  ones  would  thus  by  the  loss  of their  corpuscles  become  positively  electrified, while  the  corpuscles  driven  off  would,  if  the atoms  were  not  too  electro-positive  to  be  able  to retain  a  negative  charge  even  when  in  the  gase- ous state,  tend  to  find  a  home  on  the  more  slowly moving  atoms.  Thus,  some  of  the  atoms  would get  positively,  others  negatively  electrified,  and those  with  changes  of  opposite  signs  would  com- bine to  form  a  diatomic  molecule.  This  argu- ment would  not  apply  to  very  electro-positive gases.  These  we  should  not  expect  to  form  mole- cules, but  since  there  would  be  many  free  cor- puscles in  the  gas  we  should  expect  them  to possess  considerable  electrical  conductivity.
 


==CHAPTER  VI==
==CHAPTER  VI==

Revision as of 21:44, 11 August 2025

ELECTRICITY AND MATTER


"

J. J. THOMSON, D.Sc., LL.D., PH.D., F.R.S.

""FELLOW OF TRINITY COLLEGE, CAMBRIDGE; CAVENDISH PROFESSOR OF EXPERIMENTAL PHYSICS, CAMBRIDGE


WITH DIAGRAMS


NEW YORK

CHARLES SCRIBNER'S SONS 1904


COPYRIGHT, 1904 BY YALE UNIVERSITY

Published, March, 1904


THE SILLIMAN FOUNDATION.

In the year 1883 a legacy of eighty thousand dollars was left to the President and Fellows of Yale College in the city of New Haven, to be held in trust, as a gift from her children, in memory of their beloved and honored mother Mrs. Hepsa Ely Silliinan.

On this foundation Yale College was requested and directed to establish an annual course of lectures de- signed to illustrate the presence and providence, the wisdom and goodness of God, as manifested in the natural and moral world. These were to be designated as the Mrs. Hepsa Ely Silliinan Memorial Lectures. It was the belief of the testator that any orderly presenta- tion of the facts of nature or history contributed to the end of this foundation more effectively than any attempt to emphasize the elements of doctrine or of creed; and he therefore provided that lectures on dog- matic or polemical theology should be excluded from the scope of this foundation, and that the subjects should be selected rather from the domains of natural science and history, giving special prominence to astronomy, chemistry, geology, and anatomy.

It was further directed that each annual course should be made the basis of a volume to form part of a series constituting a memorial to Mrs. Sillimau. The memo- rial fund came into the possession of the Corporation of Yale University in the year 1902; and the present volume constitutes the first of the series of memorial lectures.


PREFACE

In these Lectures given at Yale University in May, 1903, I have attempted to discuss the bear- ing of the recent advances made in Electrical Science on our views of the Constitution of Matter and the Nature of Electricity; two questions which are probably so intimately connected, that the solution of the one would supply that of the other. A characteristic feature of recent Electri- cal Researches, such as the study and discovery of Cathode and Rontgen Rays and Radio-active Substances, has been the very especial degree in which they have involved the relation between Matter and Electricity.

In choosing a subject for the Silliman Lectures, it seemed to me that a consideration of the bear- ing of recent work on this relationship might be suitable, especially as such a discussion suggests multitudes of questions which would furnish ad- mirable subjects for further investigation by some of my hearers.

Cambridge, Aug., 1903.

J. J. THOMSON.


CONTENTS

CHAPTER- I

CHE ELECTRIC OF FORCE 1


PAGE

REPRESENTATION OF THE ELECTRIC FIELD BY LINES


CHAPTER II ELECTRICAL AND BOUND MASS 30

CHAPTER III

EFFECTS DUE TO THE ACCELERATION OF FARADAY TUBES 68

CHAPTER IV THE ATOMIC STRUCTURE OF ELECTRICITY ... 71

CHAPTER V

THK CONSTITUTION OF THE ATOM 90

CHAPTER VI

llAUIO-ACTIVITY AND RADIO-ACTIVE SUBSTANCES . . 140


ELECTRICITY AND MATTER

CHAPTER I

Thomson 1904/Chapter 1

CHAPTER II

ELECTRICAL AND BOUND MASS.

Thomson 1904/Chapter 2

CHAPTER III

EFFECTS DUE TO ACCELERATION OF THE FARADAY TUBES

Thomson 1904/Chapter 3

CHAPTER IV

THE ATOMIC STRUCTURE OF ELECTRICITY

Thomson 1904/Chapter 4

CHAPTER V

CONSTITUTION OF THE ATOM

Thomson 1904/Chapter 5

CHAPTER VI

RADIO-ACTIVITY AND RADIO-ACTIVE SUB- STANCES

IN 1896 Becquerel discovered that uranium and its salts possess the power of giving out rays which, like Rontgen and cathode rays, affect a photographic plate, and make a gas through which they pass a conductor of electricity. In 1898 Schmidt discovered that thorium possesses similar properties. This power of emitting rays is called radio-activity, and substances which possess the power are said to be radio-active.

This property of uranium led to a careful ex- amination of a large number of minerals contain- ing this substance, and M. and Mme. Curie found that some of these, and notably some specimens of pitch-blende, were more radio-active than equal volumes of pure uranium, although only a fraction of these minerals consisted of uranium. This in- dicated that these minerals contained a substance or substances much more radio-active than uran- ium itself, and a systematic attempt was made to


RADIO-ACTIVE SUBSTANCES 141

isolate these substances. After a long investigation, conducted with marvellous skill and perseverance, M. and Mme. Curie, with the collaboration of MM. Bemont and Debierne, succeeded in establishing the existence of three new radio-active substances in pitch-blende : radium associated with the ba- rium in the mineral, and closely resembling it in its chemical properties ; polonium associated with the bismuth, and actinium with the thorium. They succeeded in isolating the first of these and deter- mined its combining weight, which was found to be 225. Its spectrum has been discovered and exam- ined by Demarcay. Neither polonium nor actinium has yet been isolated, nor have their spectra been observed. The activity of polonium has been found to be fugitive, dying away in some months after its preparation.

These radio-active substances are not confined to rare minerals. I have lately found that many specimens of water from deep wells contain a radio-active gas, and Elster and Geitel have found that a similar gas is contained in the soil.

These radio-active substances may be expected to be of the greatest possible assistance in the task of investigating problems dealing with the nature of the atom, and with the changes that go on in


142 ELECTRICITY AND MATTER

the atom from time to time. For the properties possessed by these substances are so marked as to make the detection of exceedingly minute quanti- ties of them a matter of comparative ease. The quantity of these substances which can be detected is to the corresponding amount of the other ele- ments which have to be detected by the ordinary methods of chemical analysis, in the proportion of a second to thousands of years. Thus, changes which would have to go on for almost geological epochs with the non-radio-active substances, be- fore they became large enough to be detected, could with radio-active substances prove appreci- able effects in the course of a few hours.

Character of the Radiation

Rutherford found that the radiation from uran ium, and it has subsequently been found that the same is true for thorium and radium, is made up of three distinct types which he calls the a, /3, and y radiations.

The a radiation is very easily absorbed, being unable to penetrate more than a few millimetres of air at atmospheric pressure, the /8 radiation is much more penetrating, while the y radiation is the most penetrating of all. Investigations of the


RADIO-ACTIVE SUBSTANCES 143

effects of magnetic and electric forces on these three types of radiation have shown that they are of entirely different characters. Becquerel showed that the ft rays were deflected by electric and mag- netic forces, the direction of the deflection show- ing that the rays carried a charge of negative elec- tricity. He determined, using the method described

in Chapter IV, the value of —, the ratio of the

m

charge to the mass of the carriers of the negative electricity ; he found that it was about 10T, and that the velocity for some of the rays was more than two- third s that of light. He thus proved that the ft rays consisted of corpuscles travelling at prodig- ious speeds.

The a rays are not nearly so easily deflected as the ft rays, but Rutherford has recently shown that they can be deflected, and the direction of deflection shows that they carry a, positive charge. He finds, and his measurements have been con

firmed by Des Coudres, that the ratio of — is 6 X

m

103, and the velocity of these particles is 2 X 10* centimetres per second. The value of — shows that the carriers of the positive electrification have


!44 ELECTRICITY AND MATTER

masses comparable with those of ordinary atoms ;

thus — for hydrogen is 104 and for helium 2.5 X m

103. The very high velocity with which these are shot out involves an enormous expenditure of en- ergy, a point to which we shall return later. One of the most interesting things about this result is

that the value of — shows that the atoms shot off m

are not the atoms of radium, indicating either that radium is a compound containing lighter elements or else that the atom of radium is disintegrating

into such elements. The value of — for the a

m

rays obtained by Rutherford and Des Coudres suggests the existence of a gas heavier than hy- drogen but lighter than helium. The y rays, as far as we know, are not deflected either by mag- netic or electric forces.

There is considerable resemblance between a radio-active substance and a substance emitting secondary radiation under the influence of Ront- gen rays : the secondary radiation is known to contain radiation of the ($ and y types ; and as part of the radiation is exceedingly easily absorbed, being unable to penetrate more than a millimetre or so of air at atmospheric pressure, it is possible


RADIO-ACTIVE SUBSTANCES 145

that closer investigation may show that a rays, i.e.j positively electrified particles, are present also. This analogy raises the question as to whether there may not, in the case of the body struck by the Rbntgen rays, be a liberation of energy such as we shall see occurs in the case of the radio-active substances, the energy emitted by the radiating substances being greater than the energy in the Rontgen rays falling upon it ; this excess of energy being derived from changes taking place in the atoms of the body exposed to the Rontgen rays. This point seems worthy of investigation, for it might lead to a way of doing by external agency what radio-active bodies can do spontane- ously, i.e., liberate the energy locked up in the atom.

Emanation from Radio-Active Substances

Rutherford proved that thorium emits some- thing which is radio-active and which is wafted about by currents of air as if it were a gas ; in order to avoid prejudging the question as to the physical state in which the substance given off by radium exists, Rutherford called it the " emana- tion." The emanation can pass through water or the strongest acid and can be raised to tempera-


146 ELECTRICITY AND MATTER

tures at which platinum is incandescent without suffering any loss of radio-activity. In this inertness it resembles the gases argon and helium, the latter of which is almost always found associated with thorium. The radio-activity of the thorium emana- tion is very transient, sinking to half its value in about one minute.

The Curies found that radium also gives off a radio-active emanation which is much more persistent than that given off by thorium, taking about four days to sink to half its activity.

There seems every reason for thinking that those emanations are radio-active matter in the gaseous form ; they can be wafted from one place to another by currents of air  ; like a gas they dif- fuse through a porous plug at a rate which shows that their density is very high. They diffuse gradually through air and other gases. The coeffi- cient of diffusion of the radium emanation through air has been measured by Rutherford and Miss Brooks and they concluded that the density of the emanation was about eighty. The emanation of radium has been liquefied by Rutherford and Soddy ; and I have, by the kindness of Professor Dewar, been able to liquefy the radio-active gas found in water from deep wells, which very


RADIO-ACTIVE SUBSTANCES 147

closely resembles the emanation and is quite possibly identical with it. In short the emana- tions seem to satisfy every test of the gaseous state that can be applied to them. It is true that they are not capable of detection by any chemical tests of the ordinary type, nor can they be detected by spectrum analysis, but this is only because they are present in very minute quantities — quantities far too small to be detected even by spectrum analysis, a method of detection which is exceedingly rough when compared with the elec- trical methods which we are able to employ for radio-active substances. It is not, I think, an ex- aggeration to say that it is possible to detect with certainty by the electrical method a quantity of a radio-active substance less than one-hundred-thou- sandth part of the least quantity which could be detected by spectrum analysis.

Each portion of a salt of radium or thorium is giving off the emanation, whether that portion be on the inside or the outside of the salt; the emanation coming from the interior of a salt, how- ever, does not escape into the air, but gets entangled in the salt and accumulates. If such a radio- active salt is dissolved in water, there is at first a great evolution of the emanation which has been


148 ELECTRICITY AND MATTER

stored up in the solid salt. The emanation can be extracted from the water either by boiling the water or bubbling air through it. The stored up emanation can also be driven off from salts in the solid state by raising them to a very high tem- perature.

Induced Radio-Activity

Kutherford discovered that substances exposed to the emanation from thorium become radio-active, and the Curies discovered almost simultaneously that the same property is possessed by the emana- tion from radium. This phenomenon is called in- duced radio-activity. The amount of induced radio-activity does not depend upon the nature of the substance on which it is induced ; thus, paper becomes as radio-active as metal when placed in contact with the emanations of thorium or radium.

The induced radio-activity is especially de- veloped on substances which are negatively elec- trified. Thus, if the emanation is contained in a closed vessel, in which a negatively electrified wire is placed, the induced radio-activity is concentrated on the negatively electrified wire, and this induced activity can be detected on negatively electrified


RADIO-ACTIVE SUBSTANCES ^49

bodies when it is too weak to be detected on un- electrified surfaces. The fact that the nature of the induced radio-activity does not depend on the substance in which it is induced points to its being due to a radio-active substance which is deposited from the emanation on substances with which it comes in contact.

Further evidence of this is afforded by an ex- periment made by Miss Gates, in which the in- duced radio-activity on a fine wire was, by raising it to incandescence, driven off the wire and de- posited on the surrounding surfaces. The induced radio-activity due to the thorium emanation is very different from that due to the radium emana- tion, for whereas the activity of the thorium ema- nation is so transient that it drops to half its value in one minute, the induced radio-activity due to it takes about eleven hours to fall in the same proportion. The emanation due to radium, which is much more lasting than the thorium emanation, taking about four days instead of one minute to fall to half its value, gives rise to a very much less durable induced radio-activity, one fall, ing to half its value in about forty minutes instead of, as in the case of thorium, eleven hours. The emanation due to actinium is said only to be active


150 ELECTRICITY AND MATTER

for a few seconds, but the induced radio-activity due to it seems to be nearly as permanent as that due to radium.

Separation of the Active Constituent from Thorium

Rutherford and Soddy, in a most interesting and important investigation, have shown that the radio-activity of thorium is due to the passage of the thorium into a form which they call T h X, which they showed could be separated from the rest of the thorium by chemical means. When this separation has been effected the thorium left be- hind is for a time deprived of most of its radio-activ- ity, which is now to be found in the T h X. The radio-activity of the thorium X slowly decays while that of the rest of the thorium increases until it has recovered its original activity. While this has been going on, the radio-activity of the Th JThas vanished. The time taken for the radio-activity of the T h X to die away to half its original value has been shown by Rutherford and Soddy to be equal to the time taken by the thorium from which the T 7i X has been separated to recover half its original activity. All these results support the view that the radio-active part of the thorium, the thorium X, is continually being produced from the


RADIO-ACTIVE SUBSTANCES 151

thorium itself ; so that if the activity of thorium X were permanent, the radio-activity of the tho- rium would continually increase. The radio-activ. ity of the thorium X, however, steadily dies away. This prevents the unlimited increase of the radio- activity of the mixture, which will reach a steady value when the increase in the radio-activity due to the production of fresh T h X is balanced by the decay in the activity of that already produced. The question arises as to what becomes of the Tli X and the emanation when they have lost their radio-activity. This dead ThX, as we may call it, is accumulating all the time in the thorium; but inasmuch as it has lost its radio-activity, we have only the ordinary methods of chemical analy- sis to rely upon, and as these are almost infinitely less delicate than the tests we can apply to radio- active substances, it might take almost geological epochs to accumulate enough of the dead TJiX to make detection possible by chemical analysis. It seems possible that a careful examination of the minerals in which thorium and radium occur might yield important information. It is remark- able that helium is almost invariably a constitu- ent of these minerals.

You will have noticed how closely, as pointed


152 ELECTRICITY AND MATTER

out by Rutherford and Soddy, the production of radio-activity seems connected with changes tak- ing place in the radio-active substance. Thus, to take the case of thorium, which is the one on which we have the fullest information,we have first the change of thorium into thorium X, then the change of the thorium X into the emanation and the substance forming the a rays. The radio- activity of the emanation is accompanied by a fur- ther transformation, one of the products being the substance which produces induced radio-activity.

On this view the substance while radio-active is continually being transformed from one state to another. These transformations may be ac- companied by the liberation of sufficient energy to supply that carried off by the rays it emits while radio-active. The very large amount of energy emitted by radio-active substances is strik- ingly shown by some recent experiments of the Curies on the salts of radium. They find that those salts give out so much energy that the absorption of this by the salt itself is sufficent to keep the temperature of the salt permanently above that of the air by a very appreciable amount — in one of their experiments as much as 1.5° C. It appears from their measurements that a gram of radium


RADIO-ACTIVE SUBSTANCES 153

gives out enough energy per hour to raise the temperature of its own weight of water from the freezing to the boiling point. This evolution of energy goes on uninterruptedly and apparently without diminution. If, however, the views we have just explained are true, this energy arises from the transformation of radium into other forms of matter, and its evolution must cease when the stock of radium is exhausted ; unless, indeed, this stock is continually being replenished by the transformation of other chemical elements into radium.

We may make a rough guess as to the probable duration of a sample of radium by combining the result that a gram of radium gives out 100 calories per hour with Rutherford's result that the a rays are particles having masses comparable with the mass of an atom of hydrogen projected with a velocity of about 2 X 109 centimetres per second  ; for let us suppose that the heat measured by the Curies is due to the bombardment of the radium salt by these particles, and to get a superior limit to the time the radium will last, let us make the assumption that the whole of the mass of radium gets transformed into the a par- ticles (as a matter of fact we know that the emana-


154 ELECTRICITY AND MATTER

tion is produced as well as the a particles). Let a? be the life in hours of a grain of radium  ; then since the gram emits per hour 100 calories, or 4.2 X 109 ergs, the amount of energy emitted by the radium during its life is x X 4.2 X 109 ergs. If jVis the number of a particles emitted in this time, m the mass of one of them in grams, v the velocity, then the energy in the a particles is J Nmv*, but this is to be equal to a? X 4.2 X 10* ergs, hence £ Nm v* = x X 4.2 X 109 ; but if the gram of radium is converted into the a particles, Nm = 1, and by Rutherford's experiments v = 2

4 V 1018 10' X 10', hence we have «  = | Jg * ^ = g

hours, or about 50,000 years.

From this estimate we should expect the life of a piece of radium to be of the order of 50,000 years. This result shows that we could not expect to detect any measurable changes in the space of a few months. In the course of its life the gram of radium will have given out about 5 X 1010 calories, a result which shows that if this energy is derived from transformations in the state of the radium, the energy developed in these transformations must be on a very much greater scale than that developed in any known chemical


RADIO-ACTIVE SUBSTANCES 155

reactions. On the view we have taken the differ- ence between the case of radium and that of or- dinary chemical reactions is that in the latter the changes are molecular, while in the case of ra- dium the changes are atomic, being of the nature of a decomposition of the elements. The example given on page (111) shows how large an amount of energy may be stored up in the atom if we re- gard it as built up of a number of corpuscles.

We may, I think, get some light on the processes going on in radium by considering the behavior of a model atom of the kind described on page 124, and which may be typified by the case of the corpuscles which when rotating with a high velocity are stable when arranged in a certain way, which arrangement becomes unstable when the energy sinks below a certain value and is succeeded by another configuration. A top spin- ning about a vertical axis is another model of the same type. This is stable when in a vertical position if the kinetic energy due to its rotation exceeds a certain value. If this energy were gradually to decrease, then, when it reached the critical value, the top would become unstable and would fall down, and in so doing would give a considerable amount of kinetic energy.


156 ELECTRICITY AND MATTER

Let us follow, then, the behavior of an atom of this type, i.e., one which is stable in one configura- tion of steady motion when the kinetic energy of the corpuscles exceeds a certain value, but be- comes unstable and passes into a different config- uration when the kinetic energy sinks below that value. Suppose now that the atom starts with an amount of kinetic energy well above the critical value, the kinetic energy will decrease in conse- quence of the radiation from the rapidly moving corpuscles ; but as long as the motion remains steady the rate of decrease will be exceedingly slow, and it may be thousands of years before the energy approaches the critical value. When it gets close to this value, the motion will be very easily disturbed and there will probably be considerable departure from the configuration for steady motion accompanied by a great increase in the rate at which kinetic energy is loss by radiation. The atom now emits a much greater number of rays and the kinetic energy rapidly approaches the critical value ; when it reaches this value the crash comes, the original configuration is broken up, there is a great decrease in the potential energy of the sys- tem accompanied by an equal increase in the kinetic energy of the corpuscles. The increase in


RADIO-ACTIVE SUBSTANCES 157

the velocity of the corpuscles may cause the dis- ruption of the atom into two or more systems, cor- responding to the emission of the a rays and the emanation.

If the emanation is an atom of the same type as the original atom, i.e., one whose configuration for steady motion depends on its kinetic energy, the process is repeated for the emanation, but in a very much shorter time, and is repeated again for the various radio-active substances, such as the induced radio-active substance formed out of the emanation.

We have regarded the energy emitted by radium and other radio-active substances as de- rived from an internal source, i.e., changes in the constitution of the atom ; as changes of this kind have not hitherto been recognized, it is desirable to discuss the question of other possible sources of this energy. One source which at once sug- gests itself is external to the radium. We might suppose that the radium obtained its energy by absorbing some form o£ radiation which is passing through all bodies on the surface of the earth, but which is not absorbed to any extent by any but those which are radio-active. This radiation must be of a very penetrating character, for radium


158 ELECTRICITY AND MATTER

retains its activity when surrounded by thick lead or when placed in a deep cellar. We are familiar with forms of Rontgen rays, and of rays given out by radium itself, which can produce appreci- able effects after passing through several inches of lead, so that the idea of the existence of very pene- trating radiation does not seem so improbable as it would have done a few years ago. It is interest- ing to remember that very penetrating radiation was introduced by Le Sage more than a century ago to explain gravitation. Le Sage supposed that the universe was thronged with exceedingly small particles moving with very high velocities. He called these ultra-mundane corpuscles and as- sumed that they were so penetrating that they could pass through masses as large as the sun or the planets without suffering more than a very slight absorption. They were, however, absorbed to a slight extent and gave up to the bodies through which they passed a small fraction of their momentum. If the direction of the ultra- mundane corpuscles passing through a body were uniformly distributed, the momentum communi- cated by them to the body would not tend to move it in one direction rather than another, so that a body A alone in the universe and exposed to


RADIO-ACTIVE SUBSTANCES 159

bombardment by Le Sage's corpuscles would re- main at rest ; if, however, there is a second body B in the neighborhood of A, B will shield off from A some of the corpuscles moving in the direction B A  ; thus, A will not receive as much momentum in this direction as it did when it was alone in the field, but in the latter case it only re- ceived enough momentum in this direction to keep it in equilibrium ; hence, when B is present, the momentum in the opposite direction will get the upper hand so that A will move in the direction, A B, i.e., will be attracted to B. Maxwell pointed out that this transference of momentum from Le Sage's corpuscles to the body through which they were passing involved the loss of kinetic energy by the corpuscles  ; and that if the loss of momen- tum were sufficient to account for gravitation, the kinetic energy lost by the ultra-mundane cor- puscles would be sufficient, if converted into heat, to keep the gravitating body white hot. The fact that all bodies are not white hot was urged by Maxwell as an argument against Le Sage's theory. It is not necessary, however, to suppose that the energy of the corpuscles is transformed into heat ; we might imagine it transformed into a very penetrating radiation which might escape


1(50 ELECTRICITY AND MATTER

from the gravitating body. A simple calculation will show that the amount of kinetic energy transformed per second in each gram of the gravitating body must be enormously greater than that given out in the same time by one gram of radium.

"We have seen in the first chapter that waves of electric and magnetic force possess momentum in their direction of propagation; we might there- fore replace Le Sage's corpuscles by very pene- trating Rontgen rays. Those, if absorbed, would give up momentum to the bodies through which they pass, and similar consideration to those given by Le Sage would show that two bodies would attract each other inversely as the square of the distance between them. If the absorption of these waves per unit volume depended only upon, and was proportional to, the density, the attraction between the bodies would be directly proportional to the product of their masses. It ought to be mentioned that on this view any changes in gravitation would be propagated with the velocity of light; whereas, astronomers be- lieve they have established that it travels with a very much greater velocity.

As in the case of Le Sage's corpuscles, the loss


RADIO-ACTIVE SUBSTANCES IQ±

of momentum by the Rontgen rays would be ac- companied by a loss of energy; for each unit of momentum lost v units of energy would be lost, v being the velocity of light. If this energy were transformed into that of rays of the same type as the incident rays, a little reflection will show that he absorption of the rays would not produce gravitational attraction. To get such attraction the transformed rays must be of a more pene- trating type than the original rays. Again, as in the case of Le Sage's corpuscles, the absorp- tion of energy from these rays, if they are the cause of gravitation, must be enormous — so great that the energy emitted by radium would be but an exceedingly small fraction of the energy being transformed within it. From these considerations I think that the magnitude of the energy radiated from radium is not a valid argument against the energy being derived from radiation. The reason which induces me to think that the source of the energy is in the atom of radium itself and not ex- ternal to it is that the radio-activity of substances is, in all cases in which we have been able to local- ize it, a transient property. No substance goes on being radio-active for very long. It may be asked how can this statement be reconciled with the fact


162 ELECTRICITY AND MATTER

that thorium and radium keep up their activity without any appreciable falling off with time. The answer to this is that, as Rutherford and Soddy have shown in the case of thorium, it is only an exceedingly small fraction of the mass which is at any one time radio-active, and that this radio-active portion loses its activity in a few hours, and has to be replaced by a fresh supply from the non-radio-active thorium. Take any of the radio-active substances we have described, the ThX, the emanations from thorium or radium, the substance which produces induced radio- activity, all these are active for at the most a few days and then lose this property. This is what we should expect on the view that the source of the radio-activity is a change in the atom ; it is not what we should expect if the source were ex- ternal radiation.

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