1321. Wood-ashes. When wood, or, indeed, any vegetable matters are burnt, various chemical changes are produced, provided they are exposed to the action of the air while combustion is taking place. When this is the case, as Liebig observes, 'the carbon of these substances is converted into carbonic acid, their hydrogen into water, their nitrogen is set at liberty in the form of ammonia, and their sulphur assumes the form of sulphuric acid, so that at last nothing remains except the mineral ingredients of these substances in the form of ashes.' (Liebig's Chemistry, &c., 4th ed., p. 175.) These ashes, therefore, contain all the salts required for the food of plants.