Other evidence may be sought for in the spectrum of argon, which was carefully examined by Mr. Crookes. It consists of a great number of lines, extending all through the spectrum, from far down in the red to far beyond the visible violet; the invisible lines were examined by the aid of photography, for ultraviolet light, although invisible to the eye, impresses a photographic plate. The most striking feature of this spectrum is the change which can be produced in it by altering the intensity of the electric discharge which is passed through the tube containing argon at a low pressure. By interposing a Leyden jar between the secondary terminals of the induction-coil from which sparks are taken through the gas, the colour of the light in the tube changes from a brilliant red to an equally brilliant blue. A large number of lines in the red spectrum disappear, on interposing the jar, while many lines in the blue-green, blue, and violet part of the spectrum, invisible before, shine out with great brilliancy. There is no other gas in which a similar alteration of intensity of discharge produces such a marked difference, although in many gases, supposed to be simple substances, similar changes may be produced. So far as we know at present, however, such a change cannot be definitely ascribed to the presence of a mixture of two elements, although it is in itself a very remarkable phenomenon.
On the other hand, Professors Runge and Paschen, in a paper communicated to the Royal Academy of Science of Berlin in July 1895, have adduced reasons for concluding that helium, the gas from clèveite, is a mixture; it appears to show lines belonging to two spectra, each series of lines exhibiting certain regularities. But this, although an important conclusion in itself, has no direct bearing on the question of the simplicity of argon.
One method of separating the constituents of a mixture is by taking advantage of their different solubilities in water, or in some other appropriate solvent. And as argon was found to have the solubility of 4 volumes in 100 of water, while helium is very sparingly soluble, only 0·7 volume per 100, it is not unreasonable to suppose that, if argon consisted of a mixture of elements in argon, one should be more soluble than another. Exhaustive experiments in this direction have still to be carried out; but Lord Rayleigh has made experiments which render it very improbable that any separation into its constituents, if it be a mixture, can be thus effected. Wishing to ascertain if there were any helium in the air, he shook up atmospheric argon with water, until a very small fraction remained undissolved. The spectrum of this small residue was identical with that of the original argon, from which it would appear that this method, at least, is incapable of effecting any separation.
A completely decisive proof that argon is not a mixture has just been furnished by experiments carried out by Dr. Collie and Professor Ramsay, in which a large quantity of argon was submitted to fractional diffusion. From what was said on [p. 162], it will be seen that if argon consisted of a mixture of two gases of different densities, such a process should separate the mixture more or less completely into its two constituents. After a long series of diffusions, however, the density of that portion of argon passing first through the porous plug, which would have been less had any gas of lower density been present, was found to be identical with that of the last portions of gas. On the other hand, by aid of the same diffusion-apparatus, a fair separation of oxygen (density 16) from carbon dioxide (density 22) was effected, although, as the reader will observe, the densities of these two gases do not differ greatly. Hence, if argon consists of two kinds of matter they must have the same density, and hence the same molecular weights, and the difficulty is not removed. But as the spectrum of the first and last portions was the same and was identical with that of argon, this supposition is improbable.
The evidence is therefore distinctly against the supposition that argon is a mixture of two or more elements.
There is, however, another possible method of accounting for the high atomic weight of argon, which, if it could be reduced by a few units, would fall into its place after chlorine and before potassium. It is that argon consists of a mixture of many monatomic, with comparatively few diatomic, molecules. If there were only about 500 molecules of diatomic argon in every 10,000 molecules of the gas, its density, supposing it to consist entirely of monatomic molecules, would be 19, and its atomic and molecular weights 38, a number which would fit between the atomic weight of chlorine, 35·5, and that of potassium, 39·1. Several instances of this kind are known. Chlorine itself, when heated to high temperatures, changes from diatomic to monatomic molecules, and the density decreases with the change. For example, at 1000° the found density of chlorine is 27, implying a molecular weight of 54; now 54 is neither the weight of a monatomic molecule of chlorine, viz. 35·5, nor of a diatomic molecule, which is 71; but it corresponds to that of a mixture of monatomic and diatomic molecules. Here fall of temperature causes combination of monatomic molecules with each other to form diatomic molecules; and rise of temperature increases the number of monatomic molecules, at the expense of the diatomic molecules. Is there no sign of similar behaviour with argon?
It has already been mentioned that the rise of pressure of argon with rise of temperature has been carefully measured by Drs. Randall and Kuenen, and that it is quite normal; no sign of splitting has been observed. But the range of temperature was not great (it was only from 0° to 280°), and it is quite possible that the change, if there was one, was so minute as to have escaped detection. Again, a more delicate method of detecting such a change is in the measurement of the ratio of the specific heats. The most trustworthy number obtained was 1·659 for the ratio, instead of 1·667, the theoretical figure. A mixture of 5 per cent of diatomic molecules should have reduced this ratio to 1·648. Here the evidence is, however, inconclusive. But on the whole, the presumption is against the hypothesis that argon is a mixture of monatomic with diatomic molecules.
It still remains for us, therefore, to account for the fact that in the periodic table there is no place for argon, provided it be insisted on that the elements must follow each other in the numerical order of their atomic weights. If the numbers in the table actually showed regular intervals, or if there were any regularity to be detected in their differences, argon might be regarded as of wholly exceptional behaviour. But this is not so. Argon is an extreme instance of divergence, but similar divergences, though not of equal magnitude, are common.
In attempting to offer an explanation of such anomalies, it must be remembered that the question is in itself a far-reaching one; and that although argon has served to direct attention anew to the anomalies of the periodic table, yet these anomalies existed before argon was discovered. It is necessary above all things to be clear as to what is under discussion. We speak of “atomic weights,” or “atomic masses.” What is meant precisely by these expressions?
By mass, we understand that property of a body, in virtue of which, when acted on by a certain force for a certain time, it acquires a certain velocity. The product of the mass into half the velocity squared, or ½(MV)2 (where M and V stand respectively for mass and for velocity), is what is termed kinetic energy. If the mass chosen be 1 gram, and the velocity 1 centimetre per second, the unit of energy is the product; it is termed an erg. The same unit of energy, the erg, is derived by the action of unit force, termed 1 dyne, through unit length, 1 centimetre. We have thus two equations, where F and L represent force and length, Kinetic Energy = ½MV2 and Linear Energy = FL.