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Pure hydrogen is a colourless, odourless, tasteless gas. It does not support respiration, but is not poisonous. (Impure hydrogen, containing hydrogen arsenide, is poisonous.)
Hydrogen is the lightest gas known, its normal density being 0.08987 gm. per litre. The solubility in water is not greatly affected by temperature: o°, 0.0215; 10°, 0.0198; 15°, 0.0190; 20°, 0.0184.
The spectrum of hydrogen, obtained by an electrical discharge in a Geissler tube, contains four bright lines, frequently used in calibrating spectroscopes or refractometers: a red line Hα (Fraunhofer's C), 6562 A.U.; a blue line, Hγ, 4340 A.U.; a greenish-blue line, Hβ (Fraunhofer's F), 4861 A.U.; and an indigo line, Hδ 4102 A.U. (Angstrom unit = A.U. =10-10 metre = 10-8 cm. is the unit of wavelength).
Hydrogen is a good conductor of heat as compared with other gases; its conductivity is about five times that of air. Its specific heat is also abnormally high: cp = 3.4 at 0°. If a spiral of platinum wire, heated to redness by an electric current, is inserted into an inverted jar of hydrogen, the wire ceases to glow, on account of the increased loss of heat to the gas. At high temperatures a dissociation into atoms occurs H2 <=> 2H, the reaction absorbing a large amount 01 heat (about 100 k.cal. per mol.).
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