These elements are all gases at the ordinary temperature of the atmosphere, and they are consequently all to be found in atmospheric air. They are colourless, even in the liquid condition, and are devoid of smell and taste. They are very sparingly soluble in water; for example, i oo volumes of water dissolve only 4.1 volumes of argon at 15⁰. Their preparation consists, first, in the separation of the other constituents of air from them, and, second, in their separation from each other.

Air, which is a mixture, and not a compound, of nitrogen, oxygen, carbon dioxide, ammonia, water-vapour, and the gases of the helium group, is a supporter of combustion, owing to the combination of the oxygen which it contains with most other elements. Now, when air passed through a tube full of a mixture of caustic soda and lime, to remove carbon dioxide, and then through a U-tube containing sulphuric acid, to deprive it of water-vapour and ammonia, is led over red-hot copper, or over some other red-hot metal which unites with oxygen, the oxygen is retained, and nitrogen with members of the helium group alone passes on. The nitrogen can be removed in one of two ways. The first plan is due to Cavendish, who attempted to prove that atmospheric nitrogen was a homogeneous substance. He mixed atmospheric nitrogen with oxygen, and passed electric sparks through the mixture, having a little caustic soda present in the tube. Under the influence of the sparks, the nitrogen and oxygen combine, giving nitric peroxide, NO₂; this compound is absorbed by the soda, with formation of sodium nitrate and nitrite, NaNO₃ and NaNO₂. Cavendish obtained a residue of not more than one-hundred-and-twentieth of the nitrogen; and he concluded that if atmospheric nitrogen was not homogeneous, it contained only a trace of another gas. The second plan is to pass the atmospheric nitrogen over red-hot magnesium, or, better, over a mixture of magnesium powder and lime, which gives calcium ; the magnesium or the calcium unites with the nitrogen, and the inert gases pass on. A third plan is to pass air over a red-hot mixture of calcium carbide and chloride ; cyanamide of calcium and carbonate are formed, and argon escapes.

To separate these gases from each other, they are compressed into a bulb, cooled to -185⁰ by being immersed in liquid air. The argon, krypton, and xenon condense to a liquid, in which the neon and helium are dissolved. On removing the bulb from the liquid air, its temperature rises, and the helium and neon escape first, mixed with a large amount of argon. Argon distils next, and krypton and xenon remain till the last. By frequently repeating this process of " fractional distillation," the argon, krypton, and xenon can be separated from each other, and from the helium and neon which still remain mixed with each other, for both are gases at the temperature of boiling air.

To separate helium from neon, recourse must be had to liquid hydrogen. To liquefy hydrogen, the process is in principle the same as that for liquefying air, described on p. 26. The hydrogen, compressed by a pressure of 200 atmospheres, is cooled to - 205⁰ by passing through a coil of copper pipe, immersed in liquid air boiling under low pressure. On expanding, its temperature is still further lowered, and the still colder gas, in passing upwards, cools the tubes through which the compressed gas is passing. The hydrogen finally issues in the liquid state, as a colourless, mobile liquid, of the approximate temperature -250⁰. By its aid, if a mixture of neon and helium is cooled to - 250⁰, the former freezes, while the latter remains gaseous. The gaseous helium can be removed with the pump ; and the neon, after it has been warmed, may also be pumped off in a pure state.

Helium can also be prepared by heating certain specimens of pitchblende or uraninite, a mineral consisting chiefly of oxide of uranium. The gas, which appears to exist in sofd solution in the mineral, escapes; it contains a trace of argon. All these gases give very striking spectra, and that of helium was observed during the solar eclipse of 1868 in the chromosphere, or coloured atmosphere, of the sun. Although at that time it had not been discovered on the earth, the name " helium⁹⁹ was given to the bright yellow line, " D₃," which is the most characteristic of its spectrum.

As regards the relative amount of these gases contained in air, 100 volumes of air contain 0.937 volume of the mixture. By far the largest portion of this mixture is argon ; probably the volume of all the others taken together does not exceed one-four-hundredth part of that of the argon. Indeed, it may be said with truth that there is less xenon in air than there is gold in sea-water.*