192 LORD RAYLEIGH AND PROFESSOR W. RAMSAY ON ARGON, 
clear that the difference could not be accounted for by the presence of any known 
mpurity. At this stage it seemed not improbable that the hghtness of the gas 
extracted from chemical compounds was to be explained by partial dissociation of 
nitrogen molecules N, into detached atoms. In order to test this suggestion, both 
kinds of gas were submitted to the action of the silent electric discharge, with the 
result that both retained their weights unaltered. This was discouraging, and a 
further experiment pointed still more markedly in the negative direction. The 
chemical behaviour of nitrogen is such as to suggest that dissociated atoms would 
possess a higher degree of activity, and that, even though they might be formed in 
the first instance, their life would probably be short. On standing, they might be 
expected to disappear, in partial analogy with the known behaviour of ozone. With 
this idea in view, a sample of chemically-prepared nitrogen was stored for eight 
months. But, at the end of this time, the density showed no sign of increase, 
remaining exactly as at first.* 
Regarding it as established that one or other of the gases must be a mixture, 
containing, as the case might be, an ingredient much heavier or much lighter than 
ordinary nitrogen, we had to consider the relative probabilities of the various possible 
interpretations. Except upon the already discredited hypothesis of dissociation, it 
was difficult to see how the gas of chemical origin could be a mixture. To suppose 
this would be to admit two kinds of nitric acid, hardly reconcilable with the work of 
Sras and others upon the atomic weight of that substance. The simplest explanation 
in many respects was to admit the existence of a second ingredient in air from which 
oxygen, moisture, and carbonic anhydride had already been removed. The pro- 
portional amount required was not great. If the density of the supposed gas were 
double that of nitrogen, one-half per cent. only by volume would be needed ; or, if the 
density were but half as much again as that of nitrogen, then one per cent. would 
still suffice. But in accepting this explanation, even provisionally, we had to face the 
improbability that a gas surrounding us on all sides, and present in enormous 
quantities, could have remained so long unsuspected. 
The method of most universal application by which to test whether a gas is pure or 
a mixture of components of different densities is that of diffusion, By this means 
GRAHAM succeeded in effecting a partial separation of the nitrogen and oxygen of the 
air, in spite of the comparatively small difference of densities. If the atmosphere 
contain an unknown gas of anything like the density supposed, it should be possible 
to prove the fact by operations conducted upon air which had undergone atmolysis. 
If, for example, the parts least disposed to penetrate porous walls were retained, the 
“nitrogen” derived from it by the usual processes should be heavier than that 
derived in like manner from unprepared air. This experiment, although in view from 
the first, was not executed until a later stage of the inquiry ({ 6), when results were 
* Ray ecu, ‘ Proc. Roy. Soe.,’ vol. 55, p. 344, 1894. 
