AN UNDISCOVERED GAS. 249 



iron, cobalt, and nickel; palladium, rhodium, and ruthenium; and 

 platinum, iridium, and osmium. There is apparently room for a fourth 

 group of three elements in this colomn, and it may be a fifth. And 

 the discovery of such a group is not unlikely, for when this table was 

 first drawn up Professor Mendeleefif drew attention to certain gaps, 

 which have since been filled up by the discovery of gallium, germanium, 

 and others. 



The discovery of argon at once raised the curiosity of Lord Eayleigh 

 and myself as to its position in this table. With a density of nearly 20, 

 if a diatomic gas, like oxygen and nitrogen, it would follow fluorine in 

 the periodic table; and our first idea was that argon was probably a 

 mixture of three gases, all of which possessed nearly the same atomic 

 weights, like iron, cobalt, and nickel. Indeed, their names were sug- 

 gested, on this supposition, with patriotic bias, as Anglium, Scotium, 

 and Hibernium ! But when the ratio of its specific heats had, at least 

 in our opinion, unmistakably shown that it was molecularly mona- 

 tomic, and not diatomic, as at first conjectured, it was necessary to 

 believe that its atomic weight was 40, and not 20, and that it followed 

 chlorine in the atomic table, and not tiuorine. But here arises a diffi- 

 culty. The atomic weight of chlorine is 35.5, and that of potassium, 

 the next element in order in the table, is 39.1; and that of argon, 40, 

 follows, and does not precede, that of potassium, as it might be expected 

 to do. It still remains possible that argon, instead of consisting wholly 

 ot mou atomic molecules, may contain a small percentage of diatomic 

 molecules, but the evidence in favor of this supposition is, in my opin- 

 ion, far from strong. Another possibility is that argon, as at first 

 conjectured, may consist of a mixture of more than one element; but, 

 unless the atomic weight of one of the elements in the supposed mixture 

 is very high, say 82, the case is not bettered, for one of the elements in the 

 supposed trio would still have a higher atomic weight than potassium. 

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

 myself on the fractional dittusion of argon, have disproved the existence 

 of any such element with high atomic weight in argon, and, indeed, 

 have practically demonstrated that argon 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 containing uranium; although it appears to be contained 

 in others in which uranium is not present, except in traces. Among 

 those minerals are cleveite, monazite, fergusonite, and a host of similar 

 complex ndxtures, all containing rare elements, such as niobium, tan- 

 talum, yttrium, cerium, etc. The spectrum of helium is characterized 

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

 ago as 1868 by Professors Fraukland 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, and, like argon. 



