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Although an inflammable gas (gas pingue) is mentioned by Van Helmont, and the preparation, collection and combustion of hydrogen are described by Boyle, it was first adequately investigated by Cavendish in 1766.
In the free state hydrogen occurs in traces in volcanic gases; those evolved in the eruption of Mt. Pelee in 1902 contained 22.3 per cent, hydrogen. It also occurs in small cavities in rock-salt, and various minerals and rocks, such as apatite, serpentine, gneiss, blue-clay, Peter-head granite, basalt, and beryl, evolve hydrogen on heating. Meteorites composed chiefly of iron with nickel and cobalt contain hydrogen. Spectroscopic investigation shows that the outer atmosphere of the sun consists largely of hydrogen; this gas is the chief constituent of the solar prominences, which are parts of the chromosphere and are seen during total eclipse as huge red flames of incandescent gas reaching out from the sun's disc sometimes as far as 500,000 miles into space. Hydrogen is produced in certain types of fermentation of carbohydrates brought about by specific bacteria.
Hydrogen occurs chiefly in combination with other elements, especially with oxygen in the form of water, H2O, and with carbon as hydrocarbons; the gas issuing from fissures in coal often consists of nearly pure methane, CH4; more complicated hydrocarbons make up petroleum. All animal and vegetable matter and coal, contain hydrogen, and gaseous hydrogen compounds found in nature are the sulphide (H2S), phosphide (PH3), ammonia (NH3), and, in volcanic gases, the chloride (HCl), bromide (HBr), and iodide (HI). All acids and alkalies contain hydrogen.
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