Rays of Positive Electricity and Their Application to Chemical Analyses/Evolution of Helium and Neon
I have examined by the positive ray method the gases given off by a great variety of substances when they are bombarded by cathode rays,—the substances include most of the metals, a considerable number of minerals, and many metallic salts,— and have found in almost every case when cathode rays had a high speed that traces of helium were present in the gases liberated in this way. The rate of evolution of helium is generally considerably greater at the commencement of the bombardment than when this has been prolonged for some time, but in the majority of cases there seems to be a residual effect which remains even when the bombardment has lasted for a long time. The larger effect observed at first is due I think to helium absorbed by, or accumulated on, the substance bombarded; the question arises, is the more lasting evolution of this gas due to the liberation of accumulated helium lingering in the solid, or are the atoms of the elements bombarded by the cathode rays disintegrated, one of the products of disintegration being helium, or as a third alternative are there salts of helium present in the substance as impurities, and are these dissociated by the cathode rays. The first published account of the evolution of helium in vacuum tubes is that given by Sir William Ramsay ("Nature/1 July, 18, 1912); he found that when the glass of old Rontgen ray bulbs was heated, sufficient helium was given off to be detected by spectroscopic methods.
Before proceeding to consider how the various explanations of the appearance of helium may be tested, it is advisable to discuss some general aspects of the question. The difficulty of finding the origin of the helium is considerably greater than the corresponding problem for X3 owing to the presence of helium in the atmosphere. The positive ray method is so sensitive that the amount of helium in a cubic centimetre of air (which according to Ramsay is about 4 x 10-6 c.c at standard temperature and pressure) produces a strong line on the photograph, a line stronger than that found in most of the experiments described below, so that in the greater part of these experiments we are dealing with amounts of helium less than that which could be accounted for by the entrance of 1 c.c. of air into the apparatus. This source of helium has to be carefully guarded against; it is necessary, for example, to be very careful in the use of charcoal cooled by liquid air for producing the final vacuum. The cooled charcoal hardly absorbs the helium at all and thus this gas is not taken out of the vessel by this method of exhaustion. For example, if the first stages of the exhaustion were done by a water pump which takes the pressure down to a centimetre or so, and the rest of the exhaustion done by cooled charcoal, -sufficient helium and neon would be left in the vessel to give very strong lines on the positive ray photograph. It is necessary in experiments of this kind to reduce by a mercury pump the pressure to a fraction of a millimetre of mercury before applying the cooled charcoal.
We can, however, in experiments when the helium is liberated by prolonged bombardment eliminate this source of error, for if the helium and neon came from the air and not from the solid the amount of them produced would not depend on the length of the bombardment. I have checked In this way these experiments and have found that the helium is not appreciable unless the bombardment Is prolonged for an hour or so and increases in amount with the length of the bombardment, Thus if the helium comes from the air the air must have been absorbed by the substance and the helium in It liberated by the bombardment.
To test whether this was the source of the helium I bombarded soluble salts such as LiCl, NaCl, KCl, KI, RbCl, Ag NO3 which were dissolved in water and also in some cases in alcohol and then evaporated to dryness, the process being in some cases repeated several times. Salts which had been treated In this way yielded helium and in some cases neon; the yield of helium from the salts of the alkali metals and in articular from potassium was exceptionally large, KI giving a larger supply than any other of the substances I examined, with the exception of those like monazite sand which are known to contain large supplies of helium. Some of the salts have yielded apparently undiminished supplies of helium, after being dissolved and evaporated ten or twelve times. In order to see whether this process of solution and evaporation would get rid of dissolved helium, the following experiment was tried. It is well known that when the electric discharge passes through helium from aluminium electrodes, these electrodes absorb a considerable amount of helium. A piece of aluminium was divided into two portions, one half was made into electrodes of a vacuum tube filled with helium at the pressure of three or four millimetres of mercury and a current passed through the gas for two days, after this treatment the electrodes were dissolved in hydrochloric acid and evaporated to dryness. The salt thus obtained was then placed in the positive ray apparatus, bombarded by cathode rays for several hours, and a positive ray photograph of the gas given off was taken; it was found that the helium line was faint but perceptible. The other half of the aluminium which had not been near helium was then dissolved, evaporated, bombarded and the photograph taken, the intensity of the helium line in this case was but very little less than in the other, the difference not being greater than one would expect from accidental variations in the intensity of the discharge; this experiment shows, I think, that solution may be relied upon to eliminate absorbed gas.
The aluminum cathode in the tube used to bombard the substances with cathode rays might be suspected as a source of helium. If this were the case, however, the rate of production of helium would not depend upon the nature of the salt bombarded, nor would it make any difference as to whether the cathode rays hit the salt or not. As both these conditions have a great influence on the rate of production of helium we may regard this source as eliminated. In addition to the preceding considerations some of the cathodes have been in almost continuous use for months without any perceptible diminution In the rate of supply of helium.
There Is another possibility which It Is much more difficult to eliminate by means of physical experiments, though the chemical properties of the inert gases may be thought to make It very improbable; this is that helium can combine with a large number of elements and that these helium compounds exist as impurities In the salts of these elements and that these compounds of helium, present as impurities in soluble salts, are also soluble. The fact that helium is given off by nearly every substance shows that this solid compound or compounds must be extremely widely spread if it is to explain the helium production. They must also possess very special chemical properties in order to explain a large number of cases of which the following is an example: if we take AgNO3, a salt which will give some helium on bombardment, dissolve it in water, add HCl, we get a precipate of AgCl, now this silver chloride on bombardment will also give helium.
Thus if compounds of helium are present as impurities they must be precipitated by the same chemical reactions as precipitate the salts of the element with which they are mixed. It must I think be acknowledged that to explain the production of helium by the dissociation of helium compounds present as impurities obliges us to assign to helium chemical properties of a much more energetic kind than those usually assigned to it. On the other hand there are some effects connected with the appearance of helium which suggest that the source of this gas is not the whole mass of the salt but only a small fraction of it, and which would in fact be more easily explained by the presence of an impurity than on the view that the cathode rays can detach helium from any atom, of potassium say, which they happened to bombard. One effect of this kind is the very considerable variation in the amount of helium which comes off from different specimens of the same salt when bombarded under apparently similar conditions, and sometimes also of considerable variations In different salts of the same metal; thus I have always got more helium from KI than from KCl. We must not, however, lay too great stress on this for it is difficult to ensure that the bombardment by the cathode rays is of the same intensity in any two experiments; a very slight variation of the pressure of the gas in the bombardment vessel might produce large variations In the energy of the rays striking against the salt.
Another effect, which also favours the impurity view, is that the rule of evolution of helium, unlike that of X3 by the bombardment of potash, in nearly every case shows a tendency to diminish after long continued bombardment. This occurs even with salts which have been dissolved and evaporated to dryness. Experiments which are not yet completed are being made to see if the evolution of helium stops entirely after long continued bombardment. These effects could be explained and the difficulties attendant on the view that the helium comes from an impurity of the ordinary kind avoided by some such view as the following. The evolution of helium is not due solely to the bombardment, by the cathode rays. In the atoms of the ordinary elements and especially in those of the alkali metals a process is at work analogous to that which causes the expulsion of a particles from the atoms of the ordinary radioactive elements; the difference being that In the case of the ordinary elements the a particle, i.e. the helium, instead of being projected with the enormous velocity characteristic of radio-active substances, is projected with so little energy that It does not wholly escape from the parent atom. It Is loosened, so to speak, by effects which are analogous to radio-active effects and are independent of the cathode rays, the function of these rays is to detach the already loosened helium atoms. Thus it would only be certain atoms of the element which would yield helium when bombarded and when these were exhausted the supply of this gas would cease. The number of such atoms too might be expected to vary with different specimens of the same salt. Another view that might be suggested is that the helium might be formed in some such way as we have supposed X3 to be formed, i.e. by the aggregation of atoms of hydrogen. If this were the case, however, we should expect that the formation of helium would be much more pronounced when the bombarded salts contained hydrogen than when they did not; as a matter of fact, however, some of the salts which yield the largest supply of helium such as KI do not contain any hydrogen.
The view thiat helium can be got from other chemical elements raises questions of such a fundamental character that few will be prepared to accept it until every other explanation has been shown to be untenable. It would greatly strengthen the proof if we could detect the parts of the atom which remain when the helium is split off. Experiments are being made with this object but there are very considerable difficulties to be overcome. The alkali metals have so far afforded the largest supply of helium; let us consider what the residues would be in this case. If we took He from Li the residue would probably be hydrogen, now hydrogen always occurs in the positive rays and no conclusion could be drawn from its occurrence when lithium was bombarded. Next take the case of sodium; the residue after the abstraction of helium would have the atomic weight 23 - 4 = I9, which corresponds.to fluorine, an element of such energetic chemical properties that it would probably enter into chemical combination and escape detection; similar considerations apply to potassium, the residue from which might be chlorine—this again on account of Its chemical properties would be difficult to detect. If we were to take elements of much greater atomic weight the residues would have high atomic weights also and these heavy atoms are not detected by the photographic plate nearly so easily as the lighter ones like helium, so that even if there were the same number of the atoms of the residue as of the helium present In the tube the helium might show on the plate while the residues did not. The most promising elements for this purpose are light elements like carbon or beryllium whose residues would not coincide with any likely Impurities in the tube. I have tried some experiments with pure carbon but hitherto have only been able to obtain such minute quantities of helium from it that it is impossible to draw any definite conclusions.