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See also:HELIUM (from Gr. i)Xcor, the See also:sun)
, a gaseous chemical See also:element, the See also:modern See also:discovery of which followed closely on that of See also:argon (q.v.)
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The investigations of See also:Lord See also:Rayleigh and See also:Sir See also: Cleveite, samarskite and fergusonite contain a little more than monazite . The gas also occurs in See also:minute quantities in the See also:common minerals of the See also:earth's crust . In this See also:case too it is associated with radio-active See also:matter, which is almost ubiquitous . In two cases, how-ever, it has been found in the See also:absence of appreciable quantities of uranium and thorium compounds, namely in See also:beryl, and in sylvine (See also:potassium chloride) . Helium is contained almost universally in the gases which bubble up with the See also:water of thermal springs . The proportion varies greatly . In the hot springs of See also:Bath it amounts to about one-thousandth See also:part of the gas evolved . Much larger percentages have been recorded in some See also:French springs (Compt. rend., 1906, 143, p . 795, and 146, p . 435), and considerable quantities occur in some natural gas (Journ . Amer . Chem .
See also:Soc
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1524)
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See also:Strutt has suggested that helium in hot springs may be derived from the disintegration of common rocks at See also:great depths
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Helium is See also:present in the See also:atmosphere, of which it constitutes four parts in a million
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It is conspicuous by its absorption spectrum in many of the See also:
In See also:order to get rid of hydrogen, some oxygen is added to the helium, and the mixture exploded by an electric spark
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All remaining impurities, including the excess of oxygen, can then be taken out of the gas by Sir See also: The gas is in all See also:probability only mechanically retained in the minerals in which it is found . Jacquerod and See also:Perrot have found that See also:quartz-See also:glass is freely permeable to helium below a red-heat (Comps. rend., 1904, 139, p . 789) . The effect is even perceptible at a temperature as See also:low as 220 C . Hydrogen, and, in a much less degree, oxygen and nitrogen, will also permeate See also: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 See also: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 See also:molecule . The accepted atomic See also:weight is accordingly See also: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 •0093 vols. per unit See also:volume of water . The viscosity is .96 (air= I) . The spectrum of helium as observed in a See also:discharge See also:tube is distinguished by a moderate number of brilliant lines, distributed over the whole visual spectrum . The following are the approximate See also:wave-lengths of the most brilliant lines: Red . . .. 7066 Red .. . . 6678 Yellow . . . 5876 See also:Green . . . 4922 See also:Blue . . . . 4472 See also:Violet .. . . . 4026 When the discharge passes through helium at a pressure of several millimetres, the yellow line 5876 is prominent . At See also: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 behavour of a See also:constant volume gas thermometer filled with helium . For the pressure coefficient per degree, between o° and See also:roe 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 See also:special See also:advantage . At low temperatures, on the other See also:hand, they find, using an initial pressure of r000 mm., that the temperatures on the helium See also:scale are measurably higher than on the hydrogen scale, owing to the more perfectly gaseous See also:condition of helium . This difference amounts to about i1b at the temperature of liquid oxygen, and about *° at that of liquid hydrogen . The liquefaction of helium was achieved by H . Kamerlingh Onnes at See also:Leiden in 1908 . According to him its boiling point is 4.3° abs . (–268.7° C.), the density of the liquid 0.154, the See also:critical temperature 5° abs., and the critical pressure 2.3 atmospheres (Communications from the See also:Physical Laboratory at Leiden, No. ro8; see also LIQUID GASES) .
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