Ammonia, or Volatile Alkali. Although ammonia does not exist in nature in the free state, ammoniacal salts are widely distributed in the soil, and also occur in the atmosphere; they are characteristic products of the decomposition of organic substances containing nitrogen. Ammonia itself is a gaseous substance best prepared by heating ammonium chloride (sal ammoniac) with slaked lime, and collecting the product over mercury. It is a gas with a very pungent odour, which may be liquefied at 40° C. at the ordinary pressure of the atmosphere; by the rapid evaporation of this liquid ammonia may be still further obtained in white crystals. Ammonia is extremely soluble in water. It combines with acids to form ammoniacal salts, which, as a rule, are colourless and very soluble; the most important are the chloride and carbonate. In medicine it is used as an antacid and a stimulant.