(H. Eq.=1.) Hydrogen is a colourless inodorous gas, which has never been liquefied, and is but slightly soluble in water. It is the lightest form of matter known, and has a sp. gr. 0.0692. In combination with one equivalent of oxygen it forms water, and with two equivalents the peroxide of hydrogen (Ho2). The latter is a liquid of syrupy consistence, and is a very unstable compound; it possesses powerful oxidising properties, bleaches a solution of litmus, and has a somewhat metallic taste.

Therapeutics. Hydrogen possesses but little value as a remedial agent. When mixed with a certain quantity of oxygen and breathed, it imparts a peculiar shrill and sharp tone to the voice. The peroxide of hydrogen whitens the epidermis and epithelium of the tongue, and is supposed to possess some stimulant and disinfecting action upon the system, but further investigations are required to establish its value as a medicine.