598 BEPORT— 1897. 



0"1245. The spectrum of this portion does not differ in any respect from the 

 usual spectrum of helium. 



A.S re-diffusion does not alter the density or the refractivity of this gas, it is 

 right to suppose that either one definite element has now been isolated ; or that 

 if there are more elements than one present, they possess the same, or very nearly 

 the same, density and refractivity. There may be a group of elements, say three, 

 like iron, cobalt, and nickel ; but there is no proof that this idea is correct, and 

 the simplicity of the spectrum would be an argument against such a supposition. 

 This substance, forming by far the larger part of the whole amount of the gas, 

 must, in the present state of our knowledge, be regarded as pure helium. 



On the other hand, the heavier residue is easily altered in density by re-diffu- 

 sion, and this would imply that it consists of a small quantity of a heavy gas 

 mixed with a large quantity of the light gas. Repeated re-diffusion 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 re-dif- 

 fusing 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 1'64 per 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 off 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 form compounds, there does not appear to 

 be any method, other than diffusion, for isolating such a gas, if it exi.sts, and that 

 method has failed in our hands to give any evidence of the existence of such a gas. 

 It by no means follows that the gas does not exist ; the onlj' conclusion to be 

 drawn is that we have not yet stumbled on the material which contains it. In 

 fact, the haystack is too large and the needle too inconspicuous. Reference to 

 the periodic table will show that between the elements aluminium and indium 

 there occurs 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 recognised 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 accom- 

 panied in its still undiscovered source by argon and helium, it will be a work ot 

 extreme difficulty to effect a separation from these gases. 



In these remarks it has been assumed that the new gas will resemble argon 

 and helium in being indifi'erent to the action of reagents, and in iiot forming com- 

 pounds. This 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 vanadium, carbon and titanium, 

 are in each case approximately the same as that between helium and argon, viz., 36. 

 If elements further back in the periodic table be examined, it is to be noticed that 

 the differences grow less, the smaller the atomic weights. Thus, between boron 

 and scandium, the diSerence is 33 ; between beryllium (glucinum) and calcium, 

 31 ; and between lithium and potassium, 32. At the same time, we may remark 

 that the elements grow liker each other, the lower the atomic weights. Now, 

 helium and argon are very like each other in physical properties. It may be 

 fairly concluded, I think, that in so far they justify their position. Moreover, the 

 pair of elements vyhich show the smallest difference between their atomic weights 



