HELIUM, the lightest of the group of so called °inert gases° discovered in the air by Rayleigh and Ramsay, was first observed by Janssen in 1868, when, in studying the spectrum of the solar chromosphere, he found a line in the yellow (i.e., coincident with two of the sodium lines) which was not attributable to any terrestrial substance then known. Lockyer and Frankland decided it was due to the presence in the sun of a new element which they named (Greek, hellos, °(the sun*).
The first terrestrial discovery of helium was made by Hillebrand (1889, United States Geo logical Survey Bulletin), who, when working with the mineral uraninite, observed the evolu tion of a gas which gave a fluted spectrum and was attributed •by him to nitrogen. When searching for argon Ramsay found helium in the gases evolved from cleveite. Its presence in the atmosphere was first observed by Kayser (1895), who detected the helium line in the air , spectrum.
It has since been shown that helium occurs in the air in the proportion of 0.000056 per cent by weight, or about one volume of helium in 250,000 volumes of air. It also occurs in many minerals, including monazite sands, in the gases of many mineral springs and is present in com paratively large amounts in several of the nat ural gases of Canada and the United States, particularly those of Kansas and Texas. In 1907 Cady reported 1.84 per cent of helium in the natural gas at Dexter, Kan. Helium has also been found occluded in meteoric iron.
Helium is one of the products of the dis integration of radium. The rate of produc tion of helium is 0.37 cubic millimeter per day from 70 milligrams of radium chloride. Helium is also a product of the disintegration of actinium and of thorium radioactivity, which accounts for its presence in the gases given off by monazite sands.
Helium is prepared by isola tion from a mixture of other gases: (1) Nitro gen, arson, krypton, neon, xenon and helium, as obtained by removal of the oxygen from air (sec ARGON in this section) or as separated by fractional distillation of liquid air; (2) oxygen, nitrogen and helium as obtained from certain mineral springs. The gases from King's Well at Bath, England, contain 0.12 per cent of helium by volume and were used as a source of the gas by Rayleigh. This well is said to produce 1,000 litres (about 36 cubic feet) of helium per year.
Helium may be obtained in small quantities by heating certain minerals (such as cleveite, uraninite or monazite sand) either alone or with dilute sulphuric acid or an acid sulphate.
One gram of cleveite is said to yield 3.2 cubic feet of helium, and 1,000 grams of the mineral costs about $5.
The French workers Jacquerod and Perrot found that quartz is permeable to helium at 1,000° to 1,200° C. so that by sur rounding a quartz tube with impure helium at that temperature and pumping away the gas from the interior helium could be obtained free from nitrogen and other inert gases, hut it has been shown that this process is not prac ticable with some kinds of quartz. Ramsay found that at the temperature of liquid air cer tain kinds of charcoal would absorb all gases except helium and neon. The vapor pressure of neon is so different from that of helium that a systematic repetition of the process yields a pure product.
Properties.—The density of helium has been determined as from 1.98 to 2.02. Its atomic weight is taken as 3.99. It is 0.1368 times as heavy as air.
When working with the rare gases, Olszew ski obtained a sample of about 140 cubic centi meters from Lord Ramsay and tried to deter mine its critical constants, but found it im possible with the refrigeration means then at hand to reach the liquefaction point.
Dewar, in 1898, after obtaining liquid hydro gen, placed a sealed glass tube containing helium in the liquid hydrogen. A colorless liquid imme diately condensed on the sides of the tube, but the same tube in liquid air boiling under re duced pressure gave no condensation; thus the conclusion was drawn that the boiling point of helium is very close to that of hydrogen. In 1909 Onnes first liquefied helium by cooling it in solid hydrogen and then allowing it to ex pand; thus showing that the ordinary lique faction process can be applied to helium as well as to the other so-called inert gases. From 200 litres of helium gas Onnes obtained over 60 cubic centimeters of liquid helium in three hours. This liquid is colorless and its density is 0.122, which means it is the lightest liquid known. Its critical temperature is about — 268° C. (5° above absolute zero). It boils at 4.5° above ab-. solute zero and has a critical pressure of 226' atmospheres. Rapid evaporation of the liquid causes a temperature below 2.5° absolute or 270° C. but there is no indication of the formation of a solid.