TRANSACTIONS OF SECTION B. 595 



potassium. And very careful experiments, carried out by Dr. Norman Collie and 

 myself, on the fractional diffusion of argon, have disproved the existence of any 

 such element with high atomic weight in argon, and, indeed, have practically 

 demonstrated that urgon is a simple substance, and not a mixture. 



The discovery of helium has thrown a new light on this subject. Helium, it 

 will be remembered, is evolved on heating certain minerals, notably those contain- 

 ing uranium ; although it appears to be contained in others in which uranium is 

 Dot present, except in traces. Among these minerals are cleveite, monazite, 

 fergusouite, and a host of similar complex mixtures, all containing rare elements, 

 such as niobium, tantalum, yttrium, cerium, &c. The spectrum of helium is 

 characterised by a remarkably brilliant yellow line, which had been observed as 

 long ago as 1868 by Professors Frauldand and Lockyer in the spectrum of the sun's 

 chromosphere, and named ' helium' at that early date. 



The density of helium proved to be very close to 2-0, and, like argon, the ratio 

 of its specific heats showed that it, too, was a monatomic gas. Its atomic weight 

 therefore is identical with its molecular weight, viz., 4'0, and its place in the 

 periodic table is between hydrogen and lithium, the atomic weight of which 

 is 7-0. 



The difference between the atomic weights of helium and argon is thus 36, or 

 40 — 4. Now there are several cases of such a difference. For instance, in the 

 group the first member of which is fluorine we have — 



Fluorine 19 ,» . 



Chlorine 33-5 ^ 



Manganese 55 



In the oxygen group — 



19-5 



Oxygen 16 ir 



Sulphur .32 t,Q..y 



Chromium 32*3 " 



In the nitrogen group- 

 Nitrogen ... .... 14 ,_ 



Phosphorus ... ... 31 „„. 



Vanadium -JVi 



And in the carbon group — 



Carbon 12, „„ 



Silicon 28 3 ,q * 



Titanium 48 1 



These instances suffice to show that approximately the differences are 16 and 20 

 between consecutive members of the corresponding groups of elements. The total 

 diflerences between the extreme members of the short series mentioned are — 



Manganese — Fluorine 3G 



Chromium — Oxygen 36-3 



Vanadium — Nitrogen 37'4 



Titanium — Carbon . . . . . . 3G 1 



This is approximately the difference between the atomic weights of helium and 

 argon, 3'J. 



There should, therefore, be an undiscovered element between helium and argon, 

 with an atomic weight 16 units higher than that of helium, and 20 units lower 

 than that of argon, namely 20. And if this unknown element, like helium and 

 argon, should prove to consist of monatomic molecules, then its density should be 

 half its atomic weight, 10. And pushing the analogy still farther, it is to be 

 expected that this element should be as indifferent to union with other elements 

 as the two allied elements. 



QQ2 



