HELIUM (from Gr. ἥλιος, the sun), a gaseous chemical element, the modern discovery of which followed closely on that of argon (q.v.). The Investigations of Lord Rayleigh and Sir William Ramsay had shown that indifference to chemical reagents did not sufficiently characterize an unknown gas as nitrogen, and it became necessary to reinvestigate other cases of the occurrence of “nitrogen” in nature. H. Miers drew Ramsay’s attention to the work of W. F. Hillebrand, who had noticed, in examining the mineral uraninite, that an inert gas was evolved when the mineral was decomposed with acid. Ramsay, repeating these experiments, found that the inert gas emitted refused to oxidize when sparked with oxygen, and on examining it spectroscopically he saw that the spectrum was not that of argon, but was characterized by a bright yellow line near to, but not identical with, the D line of sodium. This was afterwards identified with the D3 line of the solar chromosphere, observed in 1868 by Sir J. Norman Lockyer, and ascribed by him to a hypothetical element helium. This name was adopted for the new gas.
Helium is relatively abundant in many minerals, all of which are radioactive, and contain uranium or thorium as important constituents. (For the significance of this fact see Radioactivity.) The richest known source is thorianite, which consists mainly of thorium oxide, and contains 9.5 cc. of helium per gram. Monazite, a phosphate of thorium and other rare earths, contains on the average about 1 cc. per gram. Cleveite, samarskite and fergusonite contain a little more than monazite. The gas also occurs in minute quantities in the common minerals of the earth’s crust. In this case too it is associated with radioactive matter, which is almost ubiquitous. In two cases, however, it has been found in the absence of appreciable quantities of uranium and thorium compounds, namely in beryl, and in sylvine (potassium chloride). Helium is contained almost universally in the gases which bubble up with the water of thermal springs. The proportion varies greatly. In the hot springs of Bath it amounts to about one-thousandth part of the gas evolved. Much larger percentages have been recorded in some French springs (Compt. rend., 1906, 143, p. 795, and 146, p. 435), and considerable quantities occur in some natural gas (Journ. Amer. Chem. Soc. 29, p. 1524). R. J. Strutt has suggested that helium in hot springs may be derived from the disintegration of common rocks at great depths.
Helium is present in the atmosphere, of which it constitutes four parts in a million. It is conspicuous by its absorption spectrum in many of the white stars. Certain stars and nebulae show a bright line helium spectrum.
Much the best practical source of helium is thorianite, a mineral imported from Ceylon for the manufacture of thoria. It dissolves readily in strong nitric acid, and the helium contained is thus liberated. The gas contains a certain amount of hydrogen and oxides of carbon, also traces of nitrogen. In order to get rid of hydrogen, some oxygen is added to the helium, and the mixture exploded by an electric spark. All remaining impurities, including the excess of oxygen, can then be taken out of the gas by Sir James Dewar’s ingenious method of absorption with charcoal cooled in liquid air. Helium alone refuses to be absorbed, and it can be pumped off from the charcoal in a state of absolute purity. In the absence of liquid air the helium must be purified by the methods employed for argon (q.v.). If thorianite cannot be obtained, monazite, which is more abundant, may be utilized. A part of the helium contained in minerals can be extracted by heat or by grinding (J. A. Gray, Proc. Roy. Soc., 1909, 82A, p. 301).
Properties.—All attempts to make helium enter into stable chemical union have hitherto proved unsuccessful. The gas is in all probability only mechanically retained in the minerals in which it is found. Jacquerod and Perrot have found that quartz-glass is freely permeable to helium below a red-heat (Compt. rend., 1904, 139, p. 789). The effect is even perceptible at a temperature as low as 220° C. Hydrogen, and, in a much less degree, oxygen and nitrogen, will also permeate silica, but only at higher temperatures. They have made this observation the basis of a practical method of separating helium from the other inert gases. M. Travers has suggested that it may explain the liberation of helium from minerals by heat, the gas being enabled to permeate the siliceous materials in which it is enclosed. Thorianite, however, contains no silica, and until it is shown that metallic oxides behave in the same way this explanation must be accepted with reserve.
The density of helium has been determined by Ramsay and Travers as 1.98. Its ratio of specific heats has very nearly the ideal value 1.666, appropriate to a monatomic molecule. The accepted atomic weight is accordingly double the density, i.e. approximately four times that of hydrogen. The refractivity of helium is 0.1238 (air = 1). The solubility in water is the lowest known, being, at 18.2°, only .0073 vols. per unit volume of water. The viscosity is .96 (air = 1).
The spectrum of helium as observed in a discharge tube is distinguished by a moderate number of brilliant lines, distributed over the whole visual spectrum. The following are the approximate wave-lengths of the most brilliant lines:
| Red | 7066 |
| Red | 6678 |
| Yellow | 5876 |
| Green | 4922 |
| Blue | 4472 |
| Violet | 4026 |
When the discharge passes through helium at a pressure of several millimetres, the yellow line 5876 is prominent. At lower pressures the green line 4922 becomes more conspicuous. At atmospheric pressure the discharge is able to pass through a far greater distance in helium than in the common gases.
M. Travers, G. Senter and A. Jacquerod (Phil. Trans. A. 1903, 200, p. 105) carefully examined the behaviour of a constant volume gas thermometer filled with helium. For the pressure coefficient per degree, between 0° and 100° C., they give the value .00366255, when the initial pressure is 700 mm. This value is indistinguishable from that which they find for hydrogen. Thus at high temperatures a helium thermometer is of no special advantage. At low temperatures, on the other hand, they find, using an initial pressure of 1000 mm., that the temperatures on the helium scale are measurably higher than on the hydrogen scale, owing to the more perfectly gaseous condition of helium. This difference amounts to about 110° at the temperature of liquid oxygen, and about 15° at that of liquid hydrogen.
The liquefaction of helium was achieved by H. Kamerlingh Onnes at Leiden in 1908. According to him its boiling point is 4.3° abs. (−268.7° C.), the density of the liquid 0.154, the critical temperature 5° abs., and the critical pressure 2.3 atmospheres (Communications from the Physical Laboratory at Leiden, No. 108; see also Liquid Gases).
References.—A bibliography and summary of the earlier work on helium will be found in a paper by Ramsay, Ann. chim. phys. (1898) [7], 13, p. 433. See also M. Travers, The Study of Gases (1901). (R. J. S.)