Aston 1922/Chapter 4

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 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^[1] and found to correspond with an atomic weight 20.200 (O = 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. ^[2] 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 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 comparatively 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 helium 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^[3] 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 searching investigation on the constitution of the gas by two distinct lines of attack, first attempts at separation, secondly examination by positive rays.^[4]

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.

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 construction of a simple quartz micro-balance shown in Fig. 6.^[5] The principle upon which this works is that if a sealed vacuous quartz bulb is equipoised against a solid 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 therefore 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^-6 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 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 over charcoal cooled in liquid air. The apparatus used is illustrated in the accompanying figure; the method of working was as follows:

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 operations the liquid air was removed from a and the residue it contained was pumped off completely with an Antropoff 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 intermediate 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   121.05

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 with those of the known lines due to CO and CO₂ showed, with a probability 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.^[6] 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.^[7] 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 lighter fraction showed no appreciable quantity of helium 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 and the fractionation repeated another twenty-one times, at the end of which the whole of the lightest 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 = 32). This change in density is small but it is much too marked to be ascribed to contamination 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 lines, their position and their intensity. A careful examination of the plates showed, when proper allowance 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 equally capable of carrying more than one charge.^[8] 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 consistent results, indicating that the lighter of the two corresponded 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 line 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.^[9]

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^[10] 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 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.