254 AN UNDISCOVERED GAS. 



On the other hand, the heavier residue is easily altered in density 

 by redififusion, and this wonld imply that it consists of a small quan- 

 tity of a heavy gas mixed with a large quantity of the light gas. 

 Eepeated rediffusion convinced us that there was only a very small 

 amount of the heavy gas present in the mixture. The portion which 

 contained the largest amount of heavy gas was found to have the density 

 2.275, and its refractive index was found to be 0.1333. On redift'nsing 

 this portion of gas until only a trace sufficient to fill a Pllicker's tube 

 was left, and then examining the spectrum, no unknown lines could be 

 detected; but on interposing a jar and spark gap the well-known blue 

 lines of argon became visible, and even without the jar the red lines of 

 argon and the two green groups were distinctly visible. The amount 

 of argon present, calculated from the density, was 1.04 jDer cent, and 

 from the refractivity 1.14 per cent. The conclusion had therefore to 

 be drawn that the heavy constituent of helium, as it comes oft" the 

 minerals containing it, is nothing new, but, so far as can be made out, 

 merely a small amount of argon. 



If, then, there is a new gas in what is generally termed helium, it is 

 mixed with argon, and it must be present in extremely minute traces. 

 As neither helium nor argon has been induced to ibrm compounds, 

 there does not appear to be any method other than dift'usion for isolat- 

 ing such a gas if it exists, and that method has failed in our hands to 

 give any evidence of the existence of such a gas. It by no means fol- 

 lows that the gas does not exist; the only conclusion to be drawn is 

 that we have not yet stumbled on the mateiial which contains it. In 

 fact, the haystack is too large and the needle too inconspicuous. Eefer- 

 ence to the periodic table will show that between the elements alumi- 

 num and indium there appears gallium — a substance occurring only in 

 the minutest amount on the earth's surface; and following silicon and 

 preceding tin appears the element germanium — a body which has as 

 yet been recognized only in one of the rarest of minerals, argyrodite. 

 Now, the amount of helium in fergusonite, one of the minerals which 

 yields it in reasonable 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 new gas in even more minute proportion. If, however, it 

 is accompanied in its still undiscovered source by argon and helium, it 

 will be a work of extreme difficulty to effect a separation from these 

 gases. 



In these remarks it has been assumed that the new gas will resem- 

 ble argon and helium in being indifterent to the action of reagents and 

 in not forming compounds. Tliis supposition is worth examining. In 

 considering it the analogy with other elements is all that we have to 

 guide us. 



We have already paid some attention to several triads of elements. 

 We have seen that the differences in atomic weights between the 

 elements fluorine and manganese, oxygen and chromium, nitrogen and 



