HELIUM, a gaseous element, known to be present in the atmosphere and in certain min erals, and, like argon, characterized by chemi cal inertness. It is believed that helium is the final product of the disintegration of radium. It is also the ultimate product of the radio activity of actinium and thorium. The dis covery of helium was a consequence of the dis covery of argon, and on account of the close chemical, physical and historical relations of the two, reference should be made to the article ARGoti, and to the references there given. Helium was known to exist in the sun many years before it • was discovered upon the earth. During a total eclipse of the sun, in 1868, Jannsen observed a brilliant yellow line in the spectrum of the solar chromosphere, very close to the D lines of sodium, and yet not identical with either of them. The new line was assumed to be due to a previously un known element, and in the same year Lockyer proposed the name for this hypotheti cal element, from the Greek word meaning the sun. No evidence of the existence of helium upon the earth was adduced until 1882, when an Italian scientist named Palmieri announced that he had obtained the spectrum of helium from certain of the lavas given off by Mount Vesuvius. He made no attempt to iso late the new body, however, and while it is quite possible that his observations were cor rect, he can hardly be credited with the actual discovery of helium. No further progress was made in this direction until 1895. When argon had been discovered, and its chemical inertness had been established, Mr. Miers, mineralogist of the British Museum, pointed out that the mineral cleveite had been shown to contain nitrogen gas, apparently in the free state, and made the suggestion that part of what had been assumed to be nitrogen might in reality prove to be argon. Professor William Ram say examined the gas from this source, and found that while it undoubtedly did contain argon, it also showed a brilliant yellow line, which did not appear to coincide with either of the sodium lines, though it was very close to them. He sent a specimen of the gas to Sir William Crookes for a more careful examina tion, and Crookes promptly reported that the new line was identical with the helium line.
It was therefore proved that helium, which had previously not been certainly known except as a constituent of the solar chromosphere, is also a terrestrial element. Subsequent study revealed the presence of helium in brogerite, urainite, fergusonite and several other minerals. It is given off from cleveite when that mineral is heated to about 400° F. in an exhausted tube, or when the mineral is treated with sulphuric acid, or with acid sulphate of potassium. By the heating process about half the helium con tent is secured; with the acid, all of it. All the minerals which contain any considerable quantities of helium also contain uranium, yttrium or thorium. It is not certainly known whether the helium is chemically combined with the mineral, or whether it is merely occluded by it. The latter supposition would appear to be the more probable, judging from the chemical inertness of the gas, and from the fact (pres ently to be noted) that radium appears to be generating helium continuously. Certain ob servations upon the mineral fergusonite, how ever, appear to give some color to the hypoth esis that the helium is present in a state of chemical combination. As obtained from min erals helium usually contains 10 per cent of nitrogen and a smaller per cent of argon. Helium has also been found among the gases arising from the waters of certain hot springs, notably those of Maizieres, in which the propor tion is 5.34 per cent. It occurs also in the natural gas of Kansas in the proportion of 1.84 per cent.
The presence of helium in the earth's atmos phere was established by means of subjecting the apparently pure argon that had been ob tained from this source to a process of diffu sion through a series of porous partitions of baked clay. Helium, being much lighter than argon, diffuses far more rapidly, and a mixture in which the two gases exist together may be partially separated into its constituents in this manner. The gas is present in the earth's atmos phere in the proportion of 1 volume of helium in 250,000 volumes of air.