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See also: element, the See also: modern See also: discovery of which followed closely on that of argon (q.v.)
.
The investigations of See also: Lord See also: Rayleigh and See also: Sir See also: William
See also: Ramsay had shown that indifference to chemical reagents did not sufficiently characterize an unknown See also: gas as nitrogen, and it became necessary to reinvestigate other cases of the occurrence of "nitrogen" in nature
.
H
.
Miers See also: drew Ramsay's See also: attention to the See also: work of W F
.
See also: Hillebrand, who had noticed, in examining the See also: 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 See also: oxygen, and on examining it spectroscopically he saw that the spectrum was not that of argon, but was characterized by a bright yellow See also: line near to, but not identical with, the D line of sodium
.
This was after-wards identified with the D3 line of the solar chromosphere, observed in 1868 by Sir J
.
Norman See also: Lockyer, and ascribed by him to a hypothetical element See also: 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 RADIOAcT1~1TY.) The richest known source is See also: thorianite, which consists mainly of thorium See also: oxide, and contains 9.5 cc. of helium per See also: gram
.
See also: Monazite, a phosphate of thorium and other rare earths, contains on the See also: average about r cc. per gram
.
Cleveite, samarskite and fergusonite contain a little more than monazite . The gas also occurs in minute quantities in theSee 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 (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 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
.
29, p
.
1524)
.
R
.
J
.
See also: Strutt has suggested that helium in hot springs may be derived from the disintegration of common rocks at See also: great depths
.
Helium is See also: present in the atmosphere, of which it constitutes four parts in a million
.
It is conspicuous by its absorption spectrum in many of the See also: white stars
.
Certain stars and pebulae show a bright line helium spectrum
.
Much the best
See also: practical source of helium is thorianite, a mineral imported from See also: 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 See also: carbon, also traces of nitrogen
.
In See also: 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 See also: James
See also: Dewar's ingenious method of absorption with See also: charcoal cooled in liquid air
.
Helium alone refuses to be absorbed, and it can be pumped off from the charcoal in a See also: state of absolute purity
.
In the absence of liquid air the helium must he 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
.
See also: Gray, Proc
.
See also: Roy
.
Soc., 1909, 82A, p
.
301)
.
Properties.--All attempts to make helium enter into See also: 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 andSee 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 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 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 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 Blue . . . . 4472See 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 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 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 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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