324 



7 he Homogeneity of Helium. 



■will be seen, is not far removed from those of the other pairs of 

 elements. It appears, therefore, not improbable that there should 

 be an element with atomic weight 20, resembling both argon and 

 helium in its properties. Yet it is not so certain that the middle 

 element should resemble argon and helium, for in the table given it 

 is seen that there are several examples of elements with a middle 

 place which do not resemble those at the extremes. The question is 

 perhaps best left open. 



It will be remembered that the gases evolved from a great many 

 minerals and mineral waters have been examined, and that in many 

 cases they have been found to contain helium and argon. In no 

 instance up to the present has any sample of the gases evolved on 

 heating in vacuum been found to show unknown spectrum lines. 

 The amount of argon, as proved by the account which we have just 

 given of our experiments, is very small, and in the case of the gas 

 from cleveite investigated by Langlet it is probable that argon was 

 almost completely absent, for it possessed the density 2. In mala- 

 cone, on the contrary, argon is present in larger amount than helium, 

 although neither gas is obtainable from it in large quantity. It 

 appears to us not beyond the limit of probability that in some as yet 

 uninvestigated mineral the middle member of the helium group may 

 be discovered. When it is considered that germanium, an element 

 which has been recognised only in one of the rarest of minerals, 

 argyrodite, is the middle element of the trio, silicon, germanium, and , 

 tin, of which the first and last members are common, it is surely not 

 unreasonable to hope that the middle member of the helium trio may 

 ultimately be found. The amount of helium in fergusonite, one of 

 the minerals which yields it in fair quantity, is only 33 parts by 

 weight in 100,000 of the mineral, and it is not improbable that some 

 other mineral may contain the missing gas in still more minute pro- 

 portion. If, however, it is accompanied in its still undiscovered 

 sources by argon and helium, it will probably be a work of extreme- 

 difliculty to effect its separation from these gases. 



Addendum— Since this paper was written, Professors Runge and 

 Paschen, in a communication to the British Association in August of 

 this year, have withdrawn their contention that helium is a mixture, 

 or, perhaps more correctly stated, they now ascribe to helium the 

 same complexity as that of oxygen, the spectrum of which may also 

 be arranged in two series, each consisting of three sets of lines. As 

 oxygen has not yet proved to be complex, the surmise that helium is 

 complex therefore falls to the ground. 



