When pure, ammonia is a colourless gas, capable of being liquified; of very pungent odour, the fumes producing an alkaline reaction; it forms salts with acids, but always takes an atom of basic water, and hence by most chemists these salts are regarded as containing an oxide of a hypothetical metal called ammonium (Nh4); thus sal ammoniac may be regarded as a hydrochlorate of ammonia (Nh3, Hc1) or chloride of ammonium (Nh4 Cl). Ammonia also forms direct combinations with acids, as carbonic acid, not true salts; a compound of carbonic acid and ammonia (Nh3, Co2) is perhaps present in the sesquicarbonate or so-called carbonate of the Pharmacopoeia. Gaseous ammonia is sometimes made use of therapeutically, evolved usually when thus employed, from liquor ammonias, in which it is contained.