Aston 1922/Chapter 4: Difference between revisions

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*[[Aston_1922/Appendices|Appendices]]
*[[Aston_1922/Appendices|Appendices]]


===23. Positive Ray Analysis of Neon=== 
==23. Positive Ray Analysis of Neon==  
It  is  a  curious  and
interesting  point  that  while  the  first  suggestion  of  the  possi-
bility of  the  occurrence  of  isotopes  was  obtained  from  the
rarest  of  all  substances  on  the  earth's  surface  the  radio=
active
elements  and  their  products  ;  so  the  first  result  indicating
the  possibility  of  isotopes  among  the  stable  elements  was
yielded  by  neon,  a  gas  of  which,  in  a  purified  state,  there  was
probably  less  than  one  gramme  in  existence.


Neon  is one  of the inactive  constituents  of the atmosphere,
It is a curious and interesting point that while the first suggestion of the possibility of the occurrence of isotopes was obtained from the rarest of all substances on the earth's surface the radioactive elements and their products; so the first result indicating the possibility of isotopes among the stable elements was yielded by neon, a gas of which, in a purified state, there was probably less than one gramme in existence.
in  which  it  occurs  to  the extent  of 0-00123  per  cent,  by  volume=
It  was  first isolated  by  Ramsay  and  Travers  in  1898,  and  was
accepted  as  an  elementary  monatomic  element  of the helium
group.  Its  density  was measured  with  extreme  care  by
Watson  ^  and  found  to  correspond  with  an  atomic  weight
20-200  (0  =3D  16), so  that  it  is  the  lightest  element  whose  atomic
weight  differs  from  a whole  number  in an  unmistakeable
manner.


In  the summer  of 1912  there  had  been  constructed  in the
Neon is one of the inactive constituents of the atmosphere, in which it occurs to the extent of 0-00123 per cent, by volumeIt was first isolated by Ramsay and Travers in 1898, and was accepted as an elementary monatomic element of the helium group. Its density was measured with extreme care by Watson ^ and found to correspond with an atomic weight 20-200 (0 =3D 16), so that it is the lightest element whose atomic weight differs from a whole number in an unmistakeable manner.
Cavendish  Laboratory  a  Positive  Ray  apparatus  which was  a
considerable  improvement  on  those  made  previously,  ^  The
parabolas  corresponding  to masses  differing  by  10  per cent,
could  be  clearly  resolved  and distinguished  by its  means.
Many  gases  were  submitted  to analysis  ;  but  no  results  were
obtained  which  could  not  be  accounted  for  until  in  November
of  that year  a sample  of  the  lighter  constituents  of  air  was
introduced.     In  describing  the  results  obtained  one  cannot  do


=C2=BB  Watson, J.C.S.  Trans. 1,  810,    1910.
In the summer of 1912 there had been constructed in the Cavendish Laboratory a Positive Ray apparatus which was a considerable improvement on those made previously, ^ The parabolas corresponding to masses differing by 10 per cent, could be clearly resolved and distinguished by its means. Many gases were submitted to analysis ; but no results were obtained which could not be accounted for until in November of that year a sample of the lighter constituents of air was introduced.   In describing the results obtained one cannot do


*  J. J. ThoToson^^Rays  of  Positive  EhctricUy,^. 20,
=C2=BB Watson, J.C.S. Trans. 1, 810,  1910.


33  D
* J. J. ThoToson^^Rays of Positive EhctricUy,^. 20,


33 D


34  ISOTOPES


better  than  quote  Sir  J.  J.  Thomson's  own  words  from  his
34 ISOTOPES
address  to  the  Royal  Institution  on  Friday,  January  17,
1913.


"  I  now  turn  to the photograph  of  the  lighter  constituents  ;
better than quote Sir J. J. Thomson's own words from his address to the Royal Institution on Friday, January 17, 1913.
here  we  find  the  lines  of  heUum,  of  neon  (very  strong),  of
argon, and  in  addition  there  is  a  line  corresponding  to  an
atomic  weight  22, which  cannot  be  identified  with  the  line
due  to  any  known  gas. I  thought  at  first  that  this  line,  since=


its atomic weight is one-half that of CO2, must be due to a
<blockquote>
carbonic acid molecule with a double charge of electricity,
"I now turn to the photograph of the lighter constituents; here we find the lines of heUum, of neon (very strong), of argon, and in addition there is a line corresponding to an atomic weight 22, which cannot be identified with the line due to any known gas. I thought at first that this line, since its atomic weight is one-half that of CO2, must be due to a carbonic acid molecule with a double charge of electricity, and on some of the plates a faint line at 44 could be detected. On passing the gas slowly through tubes immersed in liquid air the line at 44 completely disappeared, while the brightness of the one at 22 was not affected.
and on some of the plates a faint line at 44 could be detected=
.
On passing the gas slowly through tubes immersed in liquid
air the line at 44 completely disappeared, while the brightness
of the one at 22 was not affected.


" The origin of this line presents many points of interest ;
"The origin of this line presents many points of interest ; there are no known gaseous compounds of any of the recognised elements which have this molecular weight. Again, if we accept Mendeleef's Periodic Law, there is no room for a new element with this atomic weight. The fact that this line is bright in the sample when the neon line is extraordinarily bright, and invisible in the other when the neon is compara- tively feeble, suggests that it may possibly be a compound of neon and hydrogen, NeHg, though no direct evidence of the combination of these inert gases has hitherto been found. I have two photographs of the discharge through heUum in which there is a strong line, 6, which could be explained by the compound HeHg, but, as I have never again been able to get these lines, I do not wish to lay much stress on this point. There is, however, the possibility that we may be interpreting Mendeleef's law too rigidly, and that in the neighbourhood of the atomic weight of neon there may be a group of two or more elements with similar properties, just as in another part of the table we have the group iron, nickel, and cobalt. From the relative intensities of the 22 fine and the neon line we may conclude that the quantity of the gas giving the 22 line is only a small fraction of the quantity of neon."<blockquote>
there are no known gaseous compounds of any of the recognised
elements which have this molecular weight. Again, if we
accept Mendeleef's Periodic Law, there is no room for a new
element with this atomic weight. The fact that this line is
bright in the sample when the neon line is extraordinarily
bright, and invisible in the other when the neon is compara-
tively feeble, suggests that it may possibly be a compound of
neon and hydrogen, NeHg, though no direct evidence of the
combination of these inert gases has hitherto been found. I
have two photographs of the discharge through heUum in
which there is a strong line, 6, which could be explained by
the compound HeHg, but, as I have never again been able to
get these lines, I do not wish to lay much stress on this point.
There is, however, the possibility that we may be interpreting
Mendeleef's law too rigidly, and that in the neighbourhood of
the atomic weight of neon there may be a group of two or
more elements with similar properties, just as in another part
of the table we have the group iron, nickel, and cobalt. From
the relative intensities of the 22 fine and the neon line we may
conclude that the quantity of the gas giving the 22 line is only
a small fraction of the quantity of neon."


Other samples of gas containing neon all gave similar results.
Other samples of gas containing neon all gave similar results. By good fortune some of the purest neon in existence was also available; this had been employed by the writer and Watson
By good fortune some of the purest neon in existence was also
in some investigations on the Crookes Dark Space ^ and was actually a part of that by which the atomic weight had been determined. This sample also yielded the two separate parabolas with the same relative intensity as the others. One of the photographs taken with neon is reproduced in Plate 1 (4) .
available;   this had been employed by the writer and Watson


The last result proved that the most careful purification had not appreciably altered the intensity ratio between the lines and might at first sight appear a strong argument for the NeH2 explanation, but further study of the parabolas only added more weight to the chemical objections against the existence of such a compound. The only other alternative was a novel and revolutionary one, namely that neon could exist in two forms and that the relation between these was precisely that which had been described by Soddy a short time before as existing between the chemically inseparable radio elements.


NEON
These considerations led the author to undertake a search- ing investigation on the constitution of the gas by two distinct lines of attack, first attempts at separation, secondly examina- tion by positive rays.^
 
 
35
 
 
in  some  investigations  on  the  Crookes  Dark  Space  ^  and  was
actually  a  part  of  that  by  which  the  atomic  weight  had  been
determined.  This  sample  also  yielded  the  two  separate
parabolas  with  the  same  relative  intensity  as  the  others.  One
of  the  photographs  taken  with  neon  is  reproduced  in  Plate  1  (4) =
.
 
The  last  result  proved  that  the  most  careful  purification  had
not  appreciably  altered  the  intensity  ratio  between  the  lines
and  might  at  first  sight  appear  a  strong  argument  for  the
NeH2  explanation,  but  further  study  of  the  parabolas  only
added  more  weight  to  the  chemical  objections  against  the
existence  of  such  a  compound.  The  only  other  alternative
was  a  novel  and  revolutionary  one,  namely  that  neon  could
exist  in  two  forms  and  that  the  relation  between  these  was
precisely  that  which  had  been  described  by  Soddy  a  short
time  before  as  existing  between  the  chemically  inseparable
radio  elements.
 
These considerations led the author to undertake a search-
ing investigation on the constitution of the gas by two distinct=
 
lines of attack, first attempts at separation, secondly examina-
tion by positive rays.^
 
===24.  Apparatus  for  the  determination  of  density=== 
As
neon  is  chemically  inactive  the  most  satisfactory  proof  of  a
partial  separation  of  its  constituents  is  a  change  in  density.
 
 
Fig.  6.  Microbalance.


It  was  therefore  necessary  to  devise  some  means  of deter-
==24. Apparatus for the determination of density==
mining density accurately,  quickly  and  with  the  minimum


1  Aston  and  Watson,  Proc.  Roy.  Soc,  86A,    1912.
As neon is chemically inactive the most satisfactory proof of a partial separation of its constituents is a change in density.
^  The  neon necessary  for  this  research  was  given  by  M. Georges=


Claude  of  Paris.


Fig. 6. Microbalance.


36  ISOTOPES
It was therefore necessary to devise some means of deter- mining density accurately, quickly and with the minimum


quantity  of  gas. All  these  desiderata  were  obtained  by  the
1 Aston and Watson, Proc. Roy. Soc, 86A,  1912. ^ The  neon necessary for this research was given by M. Georges Claude of Paris.
construction  of  a  simple  quartz  micro-balance  shown  in  Fig. 6.^
The  principle  upon  which  this works  is  that  if  a  sealed
vacuous  quartz  bulb  is  equipoised  against  a  soUd  piece  of
quartz  on  a  balance  the  system  can  only  be  exactly  balanced,
at  any  predetermined  position,  when  it  is  immersed  in  a  fluid
of  an  absolutely  definite  density  ;  if  the  density  is  too  high  =
the
bulb  will  be  buoyed  up,  if  too  low  it  will  sink. We  can  there-=


fore compare  the  densities  of  a  known  and  an  unknown  gas
by  introducing  them  successively  into  the  balance  case  and
determining  the  pressures  at  which  the  system  is  exactly
balanced.


The  moving  part  of  the  balance  is  made  entirely  of  fused
36 ISOTOPES
quartz  (shown  black).  It  turns  upon  a  single  knife-edge  cut
on  a  piece  of  quartz  rod  about  0-5  mm.  thick.  To  this  rod,  a=


few  millimetres  above  the  knife-edge,  are  fused  two  others
quantity of gas. All these desiderata were obtained by the construction of a simple quartz micro-balance shown in Fig. 6.^ The principle upon which this works is that if a sealed vacuous quartz bulb is equipoised against a soUd piece of quartz on a balance the system can only be exactly balanced, at any predetermined position, when it is immersed in a fluid of an absolutely definite density ; if the density is too high the bulb will be buoyed up, if too low it will sink. We can there- fore compare the densities of a known and an unknown gas by introducing them successively into the balance case and determining the pressures at which the system is exactly balanced.
about  the  same  thickness  forming  the  arms  of the  beam. To
the end  of one  arm  is fused  a sealed vacuous quartz bulb
holding  about  0-3  c.c.  and to  the other  a  counterpoise  made  of=


a piece of rod about 2 mm. thick. The beam is supported by
The moving part of the balance is made entirely of fused quartz (shown black). It turns upon a single knife-edge cut on a piece of quartz rod about 0-5 mm. thick. To this rod, a few millimetres above the knife-edge, are fused two others about the same thickness forming the arms of the beam. To the end of one arm is fused a sealed vacuous quartz bulb holding about 0-3 c.c. and to the other a counterpoise made of a piece of rod about 2 mm. thick. The beam is supported by its knife-edge on a horizontal quartz plate and housed in a thick glass vacuum-tight case fitting as closely as possible so that its volume is a minimum. The case is connected through the capillary tube shown to a gas pipette and a pump for the introduction and removal of gas and also to a simple form of mercury manometer. The beam was adjusted during its construction so that it balanced in air at about 85 mm. pressure. In the process of adjustment the end of the counter- poise was drawn out into a fine tail ending in a small knob ; this was used as the pointer of the beam. The sensitivity of "the balance is about 10"^ mgrm., which enables the manometer to be set to one-twentieth of a millimetre with ease.
its knife-edge on a horizontal quartz plate and housed in a
thick glass vacuum-tight case fitting as closely as possible so
that its volume is a minimum. The case is connected through
the capillary tube shown to a gas pipette and a pump for the
introduction and removal of gas and also to a simple form of
mercury manometer. The beam was adjusted during its
construction so that it balanced in air at about 85 mm.
pressure. In the process of adjustment the end of the counter-
poise was drawn out into a fine tail ending in a small knob ;
this was used as the pointer of the beam. The sensitivity of
"the balance is about 10"^ mgrm., which enables the manometer
to be set to one-twentieth of a millimetre with ease.


===25. Method of using the density balance===
==25. Method of using the density balance==
About the
About the
right  volume  of  gas,  generally  known  from  previous  experience,
is  admitted  to  the  balance  case  and  the  mercury  level  in  the
manometer  slowly  raised  (increasing  the  pressure  in  the  balance
case)  until  the  bulb  rises  and  the knob  at  the  extremity  of  the=


right volume of gas, generally known from previous experience, is admitted to the balance case and the mercury level in the manometer slowly raised (increasing the pressure in the balance case) until the bulb rises and the knob at the extremity of the


1 Aston, Proc. Roy. Soc, 89A, 440,     1914.
1 Aston, Proc. Roy. Soc, 89A, 440,   1914.




Line 218: Line 90:




counterpoise appears on the field of a fixed reading microscope.=
counterpoise appears on the field of a fixed reading microscope. The pressure is then carefully adjusted until the knob reaches some definite arbitrary zero point and shows no tendency to move. The pressure is then read off. The gas is now pumped off and the same operation repeated with a gas of known density such as pure oxygen. The ratio of the densities is clearly the inverse of the pressures read, and as the latter are low the molecular weight is given direct without any correc- tions being required.
 
The pressure is then carefully adjusted until the knob reaches
some definite arbitrary zero point and shows no tendency to
move. The pressure is then read off. The gas is now pumped
off and the same operation repeated with a gas of known
density such as pure oxygen. The ratio of the densities is
clearly the inverse of the pressures read, and as the latter are=
 
low the molecular weight is given direct without any correc-
tions being required.
 
Difficulties  connected  with  temperature,  so  serious  in  density
determinations  on  the  usual  scale,  are  eliminated,  for  so  minute=
 
is  the  quantity  of  gas  (about  0-0005  grm.)  used  that  when  this=


is compressed inside the massive walls of the balance case
Difficulties connected with temperature, so serious in density determinations on the usual scale, are eliminated, for so minute is the quantity of gas (about 0-0005 grm.) used that when this is compressed inside the massive walls of the balance case thermal equilibrium is almost instantaneous. The whole operation of determining the density of a gas to 0-1 per cent, can be completed in ten minutes. Only about half a cubic centimetre of the gas is required for the operation.
thermal equilibrium is almost instantaneous. The whole
operation of determining the density of a gas to 0-1 per cent,=


can  be  completed  in  ten  minutes.  Only  about  half  a  cubic
==26. Experiments on separation by distillation==
centimetre  of  the  gas  is  required  for  the  operation.
 
===26. Experiments on separation by distillation===
The
The
first attempt at separation was made by continual fractionation
first attempt at separation was made by continual fractionation
 
 
\/      \/      \/      \/
 
 
Fia.  7.  Fractionation  Apparatus.
 
over  charcoal  cooled  in  liquid  air.  The  apparatus  used  is
illustrated  in  the  accompanying  figure  ;  the  method  of  working
was  as  follows  :
 
 
38  ISOTOPES
 
The  gas  was  admitted  in  a,  one  of  the  small  charcoal  bulbs
a,  b,  c,  d,  all  cooled  in  liquid  air.  After  a  reasonable  time  =
had
elapsed  the  first  fraction  was  pumped  off  by  lowering  mercury
in  gas-holder  A  and  opening  the  connecting  stop-cock  between
it  and  a.  After  another  interval  the  stopcock  was  turned,  the
mercury  raised  in  A  and  the  gas  forced  into  bulb  b.  The
mercury  was  next  lowered  in  both  A  and  B,  the  former  receiving=
 
the  second  fraction  from  a  while  the  latter  withdrew  the  first
fraction  of  the  gas  now  in  b.  The  fundamental  assumption  on
which  this  arrangement  was  made  was  that  at  this  stage,  if
the  vapour-pressures  of  the  gases  are  nearly  the  same,  the  gas
in  A  would  have  the  same  composition  as  that  left  in  b,  and
that  they  therefore  might  be  mixed.  This  was  done  by  raising
the  mercury,  which  not  only  drove  the  gas  from  A  into  b  but  al=
so
the  lightest  fraction  from  B  into  c,  where  it  again  fractionated,=
 
each  process  driving  the  lower  boiling  gas  forward  and  keeping
the  higher  back.
 
The  apparatus  may  contain  any  number  of  units,  the  whole
system  being  made  cyclical  and  continuous  by  joining  the
charcoal  bulb  at  one  end  with  the  gas-holder  at  the  other.
Four  such  units  were  actually  employed,  and  after  four  opera-
tions the  liquid  air  was  removed  from  a  and  the  residue  it
contained  was  pumped  off  completely  with  an  AntropofE  pump
as  the  first  contribution  to  the  heaviest  fraction  ;  in  the  same=
 
way  that  in  D  was  also  pumped  off  as  that  of  the  lightest.  The=
 
bulb  a  was  then  immersed  again  in  liquid  air  and  the  process
continued.
 
After  about  two-thirds  of  the  gas  had  been  collected  in  this
way  as  light  and  heavy  fractions,  that  remaining  was  all
pumped  out  as  the  middle  fraction.  The  process  was  next
repeated  with  the  light  and  heavy  fractions  in  turn,  the  inter-=
 
mediate ones  being  combined  by  a  definite  rule.


By  this  arrangement,  which  does  many  operations  at  once,
the  small  quantity  of  helium  contained  in  the  original  gas  was
removed  in  a  remarkably  short  time,  after  which  the  neon  was
subjected  to  continual  fractionation  for  three  weeks.  The
gas  had  now  been  through  about  3000  fractionations  and  was
divided  into  seven  main  fractions  ;  the  densities  of  these  were=


determined  in  order  by  the  quartz  micro-balance  starting  with
\/  \/  \/  \/




NEON 39
Fia. 7. Fractionation Apparatus.


the lightest,  the figures  for  the  pressures  giving  the  same  zero=
over charcoal cooled in liquid air. The apparatus used is illustrated in the accompanying figure ; the method of working was as follows :


as  oxygen  at  76-35  were  as  follows :


(1)  (2)  (3)  (4)  (5)  (6)  (7)
38 ISOTOPES


121-05      120-95      121-05      120-90      121-00      121-05      12=
The gas was admitted in a, one of the small charcoal bulbs a, b, c, d, all cooled in liquid air. After a reasonable time had elapsed the first fraction was pumped off by lowering mercury in gas-holder A and opening the connecting stop-cock between it and a. After another interval the stopcock was turned, the mercury raised in A and the gas forced into bulb b. The mercury was next lowered in both A and B, the former receiving the second fraction from a while the latter withdrew the first fraction of the gas now in b. The fundamental assumption on which this arrangement was made was that at this stage, if the vapour-pressures of the gases are nearly the same, the gas in A would have the same composition as that left in b, and that they therefore might be mixed. This was done by raising the mercury, which not only drove the gas from A into b but also the lightest fraction from B into c, where it again fractionated, each process driving the lower boiling gas forward and keeping the higher back.
105


The mean  of these, 121-00, gives  a molecular  weight  of  20-19,
The apparatus may contain any number of units, the whole system being made cyclical and continuous by joining the charcoal bulb at one end with the gas-holder at the other. Four such units were actually employed, and after four opera- tions the liquid air was removed from a and the residue it contained was pumped off completely with an AntropofE pump as the first contribution to the heaviest fraction ; in the same way that in D was also pumped off as that of the lightest. The bulb a was then immersed again in liquid air and the process continued.
which  is  identical  within  experimental  error  with the accepted
one  of 20-200  determined  by  Watson. It  was evident  that  no
appreciable  separation  had  been  achieved.


A  positive  ray  photograph  was  taken  of the two  extreme
After about two-thirds of the gas had been collected in this way as light and heavy fractions, that remaining was all pumped out as the middle fraction. The process was next repeated with the light and heavy fractions in turn, the inter- mediate ones being combined by a definite rule.
fractions and  this  showed  no  appreciable  change  in  the relative
intensity  of  the  two  parabolas. It  was however  a  very  good
one  for  the purpose  of  measurement  and a  careful  comparison
of  their  displacements  Mdth  those  of  the  known  Unes  due  to
CO  and  CO2  showed, with  a  probabihty  almost  amounting  to
certainty,  that  the atomic  weight  of  the  lighter  was  not  as
great  as  20-20.


Encouraged  by  this evidence  it  was decided  to make  a
By this arrangement, which does many operations at once, the small quantity of helium contained in the original gas was removed in a remarkably short time, after which the neon was subjected to continual fractionation for three weeks. The gas had now been through about 3000 fractionations and was divided into seven main fractions ; the densities of these were determined in order by the quartz micro-balance starting with
further  attempt  at  separation  by the method  of  fractional
diffusion.


===27.  Experiments  on  separation  by  diffusion===
The  first
apparatus  used  was  much  the  same  as  that  described  by
Ramsay  and  CoUie  in  their  work  on  the  diffusion  of  argon  and
helium.^  The  diffusion  was  carried  out  at  a  low  pressure  and
the  plug  was  made  of  two  short  lengths  of  clay  pipe  in  series.=


NEON 39


The  method  of  fractionation  was  that  described  by  Travers.*
the lightest, the figures for the pressures giving the same zero as oxygen at 76-35 were as follows :
About  100  c.c.  of  neon  was  divided  first  into  seven  and  later
into  eight  fractions.  The  complete  series  of  fractionations
was  repeated  fifteen  times,  after  which  the two  extreme
fractions  were  roughly  purified  over  charcoal  and  their  densities
measured.  These  indicated  a  difference  of  about  a  half  per
cent., a  very  hopeful  result  moreover  the fighter  fraction
showed  no  appreciable  quantity  of  heUum  even  when  analysed
by  the method  of  positive  rays  which  is  much  more  delicate
than  the spectroscope  for  this  purpose.


The  extremely  laborious  process  was  again  taken  in  hand
(1) (2) (3) (4) (5) (6) (7)


1  Ramsay  and  Collie,  Proc.  Roy.  Soc.  60A,  206,    1896.
121-05    120-95  121-05  120-90  121-00  121-05  12105
*  Travers,  A  Stvdy  of  Gases,  p.  289.


The mean of these, 121-00, gives a molecular weight of 20-19, which is identical within experimental error with the accepted one of 20-200 determined by Watson. It was evident that no appreciable separation had been achieved.


40  ISOTOPES
A positive ray photograph was taken of the two extreme fractions and this showed no appreciable change in the relative intensity of the two parabolas. It was however a very good one for the purpose of measurement and a careful comparison of their displacements Mdth those of the known Unes due to CO and CO2 showed, with a probabihty almost amounting to certainty, that the atomic weight of the lighter was not as great as 20-20.


and  the  fractionation  repeated  another  twenty-one  times,  at
Encouraged by this evidence it was decided to make a further attempt at separation by the method of fractional diffusion.
the end  of which  the  whole  of  the  Hghtest  fraction  was  lost  by=


a  most  unfortunate  accident. This  was the more  serious  as
==27. Experiments on separation by diffusion==
the two extreme  fractions  had  been  systematically  enlarged
The first apparatus used was much the same as that described by Ramsay and CoUie in their work on the diffusion of argon and helium.^ The diffusion was carried out at a low pressure and the plug was made of two short lengths of clay pipe in series.
with  a  view  to  fractionating  each  separately.


Despite  this  setback  the  fractionation of  the  heaviest  20  c.c.
The method of fractionation was that described by Travers.* About 100 c.c. of neon was divided first into seven and later into eight fractions. The complete series of fractionations was repeated fifteen times, after which the two extreme fractions were roughly purified over charcoal and their densities measured. These indicated a difference of about a half per cent., a very hopeful result moreover the fighter fraction showed no appreciable quantity of heUum even when analysed by the method of positive rays which is much more delicate than the spectroscope for this purpose.
was  proceeded  with.  This  was divided into five  fractions and
fractionated  ten  times. The next  lightest  fraction  to  the  one
lost  was taken, divided  into  five  parts  and  fractionated  twelve
times.  These  very  tedious  operations  were  now  brought  to  a
close  and  the two extreme fractions of  2  to  3  c.c.  each  were
purified over charcoal with  the greatest  possible  care.


The final  densities  which  further  purification  failed  to  alter
The extremely laborious process was again taken in hand
were  20-15  and  20-28  (Oxygen  =3D32).  This  change  in density
is  small  but  it  is  much  too  marked  to  be  ascribed  to  con-
tamination or  to  experimental  error.  Looked  at  in  the  fight
of  modern  knowledge  there  can  be  no  reasonable  doubt  that
partial  separation  had  been  actually  achieved.  The  extent  of
the  separation  is  about  that  to  be  expected  from  the  theoretical=


considerations  of  separation  by  diffusion  given  on  page  127.
1 Ramsay and Collie, Proc. Roy. Soc. 60A, 206,  1896.
A spectroscopic  examination  of these  two  fractions  showed  no
* Travers, A Stvdy of Gases, p. 289.
appreciable  difference  between  them.


These  results  were  announced  at  the  meeting  of  the  British
Association  at  Birmingham  in  1913  and  at  the  same  time  the
evidence  afforded  by  the  positive  ray  photographs  discussed.
This  is  available  from  three  distinct  considerations :  the
character  of  the  fines,  their  position  and  their  intensity.
A  careful  examination  of  the  plates  showed,  when  proper
aUowance  had  been  made  for  difference  of  intensity,  that  the
two  parabolas  had  characteristics  identical  with  one  another.
Both  were  prolonged  towards  the  vertical  axis  showing  that
the  particles  causing  them  were  equaUy  capable  of  carrying
more  than  one  charge.^  Now  up  to  that  time  no  cases  of
multiple  charges  had  been  found  to  occur  on  molecules,  but
only  on  atoms.  One  was  therefore  led  to  infer  that  both
lines  were  due  to  elements.


Measurements  of  the  position  of  the  parabolas  relative  to
40 ISOTOPES
those  of  CO  and  other  known  bodies  in  the  discharge  tube  gave


1  V. p.  30.
and the fractionation repeated another twenty-one times, at the end of which the whole of the Hghtest fraction was lost by a most unfortunate accident. This was the more serious as the two extreme fractions had been systematically enlarged with a view to fractionating each separately.


Despite this setback the fractionation of the heaviest 20 c.c. was proceeded with. This was divided into five fractions and fractionated ten times. The next lightest fraction to the one lost was taken, divided into five parts and fractionated twelve times. These very tedious operations were now brought to a close and the two extreme fractions of 2 to 3 c.c. each were purified over charcoal with the greatest possible care.


NEON  41
The final densities which further purification failed to alter were 20-15 and 20-28 (Oxygen =3D32). This change in density is small but it is much too marked to be ascribed to con- tamination or to experimental error. Looked at in the fight of modern knowledge there can be no reasonable doubt that partial separation had been actually achieved. The extent of the separation is about that to be expected from the theoretical considerations of separation by diffusion given on page 127. A spectroscopic examination of these two fractions showed no appreciable difference between them.


consistent  results, indicating  that  the lighter  of the two corre-=
These results were announced at the meeting of the British Association at Birmingham in 1913 and at the same time the evidence afforded by the positive ray photographs discussed. This is available from three distinct considerations : the character of the fines, their position and their intensity. A careful examination of the plates showed, when proper aUowance had been made for difference of intensity, that the two parabolas had characteristics identical with one another. Both were prolonged towards the vertical axis showing that the particles causing them were equaUy capable of carrying more than one charge.^ Now up to that time no cases of multiple charges had been found to occur on molecules, but only on atoms. One was therefore led to infer that both lines were due to elements.


sponded with  an  atomic  weight  less  than  20-2,  but  the accuracy-
Measurements of the position of the parabolas relative to those of CO and other known bodies in the discharge tube gave
was  not  sufficient  to  make  this  certain.  The  relative  intensity
of the parabolas was  estimated  by  three  independent  observers
as  about  10  to 1.  Its  apparent  invariability  was  valuable
corroborative  evidence  against  the  possibility  of  the  22  Hne
being  due  to  the  presence  of other gases  in the discharge tube.=


1 V. p. 30.


===28. Second  attempt  at  separation  by  diffusion===
In
order  to  carry  out  further  diffusion  experiments  an  elaborate
automatic  diffusion  apparatus  was  devised  so  as  to  avoid  the
excessive  labour  of  working  by  hand.  This  worked  on  the
see-saw  principle  and  dealt  with  300  c.c.  of  neon  at  a  time.
It  was  started  in  1914,  but  as  it  had  little  success  in  its  ob=
ject
there  is  no  need  to  describe  it  in  detail.  It  will  be  enough  t=
o
say  that  although  it  performed  the  mechanical  operations  of
diffusion  many  thousands  of  times  in  a  satisfactory  manner  the
separation  achieved  was  exceedingly  poor  -actually  only
about  half  that  attained  previously.  This  disappointing  result
was  undoubtedly  due  to  the  mistake  made  in  designing  it  to
carry  out  the  diffusion  at  atmospheric  pressure,  for  under  these=


conditions  the  "  mixing  "  is  very  bad.^
NEON 41


When  the work  was interrupted  by the war  it  could  be  said
consistent results, indicating that the lighter of the two corre- sponded with an atomic weight less than 20-2, but the accuracy- was not sufficient to make this certain. The relative intensity of the parabolas was estimated by three independent observers as about 10 to 1. Its apparent invariability was valuable corroborative evidence against the possibility of the 22 Hne being due to the presence of other gases in the discharge tube.
that  although  the presence of two  isotopes  in neon  was  indicated=


by several hues of reasoning,  none  of  these  could  be  said  to
==28. Second attempt at separation by diffusion==
carry  absolute  conviction.


===29. The  analysis  of neon by  the  Mass-spectrograph===
In order to carry out further diffusion experiments an elaborate automatic diffusion apparatus was devised so as to avoid the excessive labour of working by hand. This worked on the see-saw principle and dealt with 300 c.c. of neon at a time. It was started in 1914, but as it had little success in its object there is no need to describe it in detail. It will be enough to say that although it performed the mechanical operations of diffusion many thousands of times in a satisfactory manner the separation achieved was exceedingly poor -actually only about half that attained previously. This disappointing result was undoubtedly due to the mistake made in designing it to carry out the diffusion at atmospheric pressure, for under these conditions the  " mixing " is very bad.^


By  the time  the  work was resumed  in  1919  the  existence  of
When the work was interrupted by the war it could be said that although the presence of two isotopes in neon was indicated by several hues of reasoning, none of these could be said to carry absolute conviction.
isotopes  among  the  products  of  radioactivity  had  been  put
beyond  aU  reasonable  doubt  by the work  on  the atomic  weight
of lead  2  and  was accepted  generally.  This  fact  automatically
increased  both  the  value  of  the  evidence  of  the  complex  nature
of neon  and  the  urgency  of its  definite  confirmation.  It  was
realised  that  separation  could only  be very  partial  at  the  best=


and  that  the  most  satisfactory  proof  would  be  afforded  by
==29. The analysis of neon by the Mass-spectrograph==
measurements  of atomic  weight  by the method  of  positive
rays.    These  would  have  to  be  so  accurate  as  to  prove  beyond=


By the time the work was resumed in 1919 the existence of isotopes among the products of radioactivity had been put beyond aU reasonable doubt by the work on the atomic weight of lead 2 and was accepted generally. This fact automatically increased both the value of the evidence of the complex nature of neon and the urgency of its definite confirmation. It was realised that separation could only be very partial at the best and that the most satisfactory proof would be afforded by measurements of atomic weight by the method of positive rays.  These would have to be so accurate as to prove beyond


1 F. p. 127.
1 F. p. 127.
  F. p. 16.
F. p. 16.




42 ISOTOPES
42 ISOTOPES


dispute that the accepted atomic weight lay between the real
dispute that the accepted atomic weight lay between the real atomic weights of the constituents, but corresponded with neither of them.
atomic weights of the constituents, but corresponded with
neither of them.


A new method of positive ray analysis was therefore worked
A new method of positive ray analysis was therefore worked out which will be described in the next chapter. This proved amply accurate enough for the purpose and the results obtained from neon, which are given in detail on page 64, show beyond any doubt that this gas is a mixture of two isotopes of atomic weights 20-00 and 22-00 respectively.
out which will be described in the next chapter. This proved
amply accurate enough for the purpose and the results obtained
from neon, which are given in detail on page 64, show beyond
any doubt that this gas is a mixture of two isotopes of atomic
weights 20-00 and 22-00 respectively.

Revision as of 21:35, 6 July 2025

Chapter IV - Neon

23. Positive Ray Analysis of Neon

It is a curious and interesting point that while the first suggestion of the possibility of the occurrence of isotopes was obtained from the rarest of all substances on the earth's surface the radioactive elements and their products; so the first result indicating the possibility of isotopes among the stable elements was yielded by neon, a gas of which, in a purified state, there was probably less than one gramme in existence.

Neon is one of the inactive constituents of the atmosphere, in which it occurs to the extent of 0-00123 per cent, by volumeIt was first isolated by Ramsay and Travers in 1898, and was accepted as an elementary monatomic element of the helium group. Its density was measured with extreme care by Watson ^ and found to correspond with an atomic weight 20-200 (0 =3D 16), so that it is the lightest element whose atomic weight differs from a whole number in an unmistakeable manner.

In the summer of 1912 there had been constructed in the Cavendish Laboratory a Positive Ray apparatus which was a considerable improvement on those made previously, ^ The parabolas corresponding to masses differing by 10 per cent, could be clearly resolved and distinguished by its means. Many gases were submitted to analysis ; but no results were obtained which could not be accounted for until in November of that year a sample of the lighter constituents of air was introduced. In describing the results obtained one cannot do

=C2=BB Watson, J.C.S. Trans. 1, 810, 1910.

  • J. J. ThoToson^^Rays of Positive EhctricUy,^. 20,

33 D


34 ISOTOPES

better than quote Sir J. J. Thomson's own words from his address to the Royal Institution on Friday, January 17, 1913.

"I now turn to the photograph of the lighter constituents; here we find the lines of heUum, of neon (very strong), of argon, and in addition there is a line corresponding to an atomic weight 22, which cannot be identified with the line due to any known gas. I thought at first that this line, since its atomic weight is one-half that of CO2, must be due to a carbonic acid molecule with a double charge of electricity, and on some of the plates a faint line at 44 could be detected. On passing the gas slowly through tubes immersed in liquid air the line at 44 completely disappeared, while the brightness of the one at 22 was not affected.

"The origin of this line presents many points of interest ; there are no known gaseous compounds of any of the recognised elements which have this molecular weight. Again, if we accept Mendeleef's Periodic Law, there is no room for a new element with this atomic weight. The fact that this line is bright in the sample when the neon line is extraordinarily bright, and invisible in the other when the neon is compara- tively feeble, suggests that it may possibly be a compound of neon and hydrogen, NeHg, though no direct evidence of the combination of these inert gases has hitherto been found. I have two photographs of the discharge through heUum in which there is a strong line, 6, which could be explained by the compound HeHg, but, as I have never again been able to get these lines, I do not wish to lay much stress on this point. There is, however, the possibility that we may be interpreting Mendeleef's law too rigidly, and that in the neighbourhood of the atomic weight of neon there may be a group of two or more elements with similar properties, just as in another part of the table we have the group iron, nickel, and cobalt. From the relative intensities of the 22 fine and the neon line we may conclude that the quantity of the gas giving the 22 line is only a small fraction of the quantity of neon."

Other samples of gas containing neon all gave similar results. By good fortune some of the purest neon in existence was also available; this had been employed by the writer and Watson in some investigations on the Crookes Dark Space ^ and was actually a part of that by which the atomic weight had been determined. This sample also yielded the two separate parabolas with the same relative intensity as the others. One of the photographs taken with neon is reproduced in Plate 1 (4) .

The last result proved that the most careful purification had not appreciably altered the intensity ratio between the lines and might at first sight appear a strong argument for the NeH2 explanation, but further study of the parabolas only added more weight to the chemical objections against the existence of such a compound. The only other alternative was a novel and revolutionary one, namely that neon could exist in two forms and that the relation between these was precisely that which had been described by Soddy a short time before as existing between the chemically inseparable radio elements.

These considerations led the author to undertake a search- ing investigation on the constitution of the gas by two distinct lines of attack, first attempts at separation, secondly examina- tion by positive rays.^

24. Apparatus for the determination of density

As neon is chemically inactive the most satisfactory proof of a partial separation of its constituents is a change in density.


Fig. 6. Microbalance.

It was therefore necessary to devise some means of deter- mining density accurately, quickly and with the minimum

1 Aston and Watson, Proc. Roy. Soc, 86A, 1912. ^ The neon necessary for this research was given by M. Georges Claude of Paris.


36 ISOTOPES

quantity of gas. All these desiderata were obtained by the construction of a simple quartz micro-balance shown in Fig. 6.^ The principle upon which this works is that if a sealed vacuous quartz bulb is equipoised against a soUd piece of quartz on a balance the system can only be exactly balanced, at any predetermined position, when it is immersed in a fluid of an absolutely definite density ; if the density is too high the bulb will be buoyed up, if too low it will sink. We can there- fore compare the densities of a known and an unknown gas by introducing them successively into the balance case and determining the pressures at which the system is exactly balanced.

The moving part of the balance is made entirely of fused quartz (shown black). It turns upon a single knife-edge cut on a piece of quartz rod about 0-5 mm. thick. To this rod, a few millimetres above the knife-edge, are fused two others about the same thickness forming the arms of the beam. To the end of one arm is fused a sealed vacuous quartz bulb holding about 0-3 c.c. and to the other a counterpoise made of a piece of rod about 2 mm. thick. The beam is supported by its knife-edge on a horizontal quartz plate and housed in a thick glass vacuum-tight case fitting as closely as possible so that its volume is a minimum. The case is connected through the capillary tube shown to a gas pipette and a pump for the introduction and removal of gas and also to a simple form of mercury manometer. The beam was adjusted during its construction so that it balanced in air at about 85 mm. pressure. In the process of adjustment the end of the counter- poise was drawn out into a fine tail ending in a small knob ; this was used as the pointer of the beam. The sensitivity of "the balance is about 10"^ mgrm., which enables the manometer to be set to one-twentieth of a millimetre with ease.

25. Method of using the density balance

About the

right volume of gas, generally known from previous experience, is admitted to the balance case and the mercury level in the manometer slowly raised (increasing the pressure in the balance case) until the bulb rises and the knob at the extremity of the

1 Aston, Proc. Roy. Soc, 89A, 440, 1914.


NEON


37


counterpoise appears on the field of a fixed reading microscope. The pressure is then carefully adjusted until the knob reaches some definite arbitrary zero point and shows no tendency to move. The pressure is then read off. The gas is now pumped off and the same operation repeated with a gas of known density such as pure oxygen. The ratio of the densities is clearly the inverse of the pressures read, and as the latter are low the molecular weight is given direct without any correc- tions being required.

Difficulties connected with temperature, so serious in density determinations on the usual scale, are eliminated, for so minute is the quantity of gas (about 0-0005 grm.) used that when this is compressed inside the massive walls of the balance case thermal equilibrium is almost instantaneous. The whole operation of determining the density of a gas to 0-1 per cent, can be completed in ten minutes. Only about half a cubic centimetre of the gas is required for the operation.

26. Experiments on separation by distillation

The first attempt at separation was made by continual fractionation


\/ \/ \/ \/


Fia. 7. Fractionation Apparatus.

over charcoal cooled in liquid air. The apparatus used is illustrated in the accompanying figure ; the method of working was as follows :


38 ISOTOPES

The gas was admitted in a, one of the small charcoal bulbs a, b, c, d, all cooled in liquid air. After a reasonable time had elapsed the first fraction was pumped off by lowering mercury in gas-holder A and opening the connecting stop-cock between it and a. After another interval the stopcock was turned, the mercury raised in A and the gas forced into bulb b. The mercury was next lowered in both A and B, the former receiving the second fraction from a while the latter withdrew the first fraction of the gas now in b. The fundamental assumption on which this arrangement was made was that at this stage, if the vapour-pressures of the gases are nearly the same, the gas in A would have the same composition as that left in b, and that they therefore might be mixed. This was done by raising the mercury, which not only drove the gas from A into b but also the lightest fraction from B into c, where it again fractionated, each process driving the lower boiling gas forward and keeping the higher back.

The apparatus may contain any number of units, the whole system being made cyclical and continuous by joining the charcoal bulb at one end with the gas-holder at the other. Four such units were actually employed, and after four opera- tions the liquid air was removed from a and the residue it contained was pumped off completely with an AntropofE pump as the first contribution to the heaviest fraction ; in the same way that in D was also pumped off as that of the lightest. The bulb a was then immersed again in liquid air and the process continued.

After about two-thirds of the gas had been collected in this way as light and heavy fractions, that remaining was all pumped out as the middle fraction. The process was next repeated with the light and heavy fractions in turn, the inter- mediate ones being combined by a definite rule.

By this arrangement, which does many operations at once, the small quantity of helium contained in the original gas was removed in a remarkably short time, after which the neon was subjected to continual fractionation for three weeks. The gas had now been through about 3000 fractionations and was divided into seven main fractions ; the densities of these were determined in order by the quartz micro-balance starting with


NEON 39

the lightest, the figures for the pressures giving the same zero as oxygen at 76-35 were as follows :

(1) (2) (3) (4) (5) (6) (7)

121-05 120-95 121-05 120-90 121-00 121-05 12105

The mean of these, 121-00, gives a molecular weight of 20-19, which is identical within experimental error with the accepted one of 20-200 determined by Watson. It was evident that no appreciable separation had been achieved.

A positive ray photograph was taken of the two extreme fractions and this showed no appreciable change in the relative intensity of the two parabolas. It was however a very good one for the purpose of measurement and a careful comparison of their displacements Mdth those of the known Unes due to CO and CO2 showed, with a probabihty almost amounting to certainty, that the atomic weight of the lighter was not as great as 20-20.

Encouraged by this evidence it was decided to make a further attempt at separation by the method of fractional diffusion.

27. Experiments on separation by diffusion

The first apparatus used was much the same as that described by Ramsay and CoUie in their work on the diffusion of argon and helium.^ The diffusion was carried out at a low pressure and the plug was made of two short lengths of clay pipe in series.

The method of fractionation was that described by Travers.* About 100 c.c. of neon was divided first into seven and later into eight fractions. The complete series of fractionations was repeated fifteen times, after which the two extreme fractions were roughly purified over charcoal and their densities measured. These indicated a difference of about a half per cent., a very hopeful result moreover the fighter fraction showed no appreciable quantity of heUum even when analysed by the method of positive rays which is much more delicate than the spectroscope for this purpose.

The extremely laborious process was again taken in hand

1 Ramsay and Collie, Proc. Roy. Soc. 60A, 206, 1896.

  • Travers, A Stvdy of Gases, p. 289.


40 ISOTOPES

and the fractionation repeated another twenty-one times, at the end of which the whole of the Hghtest fraction was lost by a most unfortunate accident. This was the more serious as the two extreme fractions had been systematically enlarged with a view to fractionating each separately.

Despite this setback the fractionation of the heaviest 20 c.c. was proceeded with. This was divided into five fractions and fractionated ten times. The next lightest fraction to the one lost was taken, divided into five parts and fractionated twelve times. These very tedious operations were now brought to a close and the two extreme fractions of 2 to 3 c.c. each were purified over charcoal with the greatest possible care.

The final densities which further purification failed to alter were 20-15 and 20-28 (Oxygen =3D32). This change in density is small but it is much too marked to be ascribed to con- tamination or to experimental error. Looked at in the fight of modern knowledge there can be no reasonable doubt that partial separation had been actually achieved. The extent of the separation is about that to be expected from the theoretical considerations of separation by diffusion given on page 127. A spectroscopic examination of these two fractions showed no appreciable difference between them.

These results were announced at the meeting of the British Association at Birmingham in 1913 and at the same time the evidence afforded by the positive ray photographs discussed. This is available from three distinct considerations : the character of the fines, their position and their intensity. A careful examination of the plates showed, when proper aUowance had been made for difference of intensity, that the two parabolas had characteristics identical with one another. Both were prolonged towards the vertical axis showing that the particles causing them were equaUy capable of carrying more than one charge.^ Now up to that time no cases of multiple charges had been found to occur on molecules, but only on atoms. One was therefore led to infer that both lines were due to elements.

Measurements of the position of the parabolas relative to those of CO and other known bodies in the discharge tube gave

1 V. p. 30.


NEON 41

consistent results, indicating that the lighter of the two corre- sponded with an atomic weight less than 20-2, but the accuracy- was not sufficient to make this certain. The relative intensity of the parabolas was estimated by three independent observers as about 10 to 1. Its apparent invariability was valuable corroborative evidence against the possibility of the 22 Hne being due to the presence of other gases in the discharge tube.

28. Second attempt at separation by diffusion

In order to carry out further diffusion experiments an elaborate automatic diffusion apparatus was devised so as to avoid the excessive labour of working by hand. This worked on the see-saw principle and dealt with 300 c.c. of neon at a time. It was started in 1914, but as it had little success in its object there is no need to describe it in detail. It will be enough to say that although it performed the mechanical operations of diffusion many thousands of times in a satisfactory manner the separation achieved was exceedingly poor -actually only about half that attained previously. This disappointing result was undoubtedly due to the mistake made in designing it to carry out the diffusion at atmospheric pressure, for under these conditions the " mixing " is very bad.^

When the work was interrupted by the war it could be said that although the presence of two isotopes in neon was indicated by several hues of reasoning, none of these could be said to carry absolute conviction.

29. The analysis of neon by the Mass-spectrograph

By the time the work was resumed in 1919 the existence of isotopes among the products of radioactivity had been put beyond aU reasonable doubt by the work on the atomic weight of lead 2 and was accepted generally. This fact automatically increased both the value of the evidence of the complex nature of neon and the urgency of its definite confirmation. It was realised that separation could only be very partial at the best and that the most satisfactory proof would be afforded by measurements of atomic weight by the method of positive rays. These would have to be so accurate as to prove beyond

1 F. p. 127. F. p. 16.


42 ISOTOPES

dispute that the accepted atomic weight lay between the real atomic weights of the constituents, but corresponded with neither of them.

A new method of positive ray analysis was therefore worked out which will be described in the next chapter. This proved amply accurate enough for the purpose and the results obtained from neon, which are given in detail on page 64, show beyond any doubt that this gas is a mixture of two isotopes of atomic weights 20-00 and 22-00 respectively.

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