Acid, the term anciently given to sour liquids, denotes, in the more restricted acceptation of modern chemistry, a Salt of Hydrogen, which is capable of exchanging the whole, or part, of the hydrogen it contains for a metal. The usual method of effecting this exchange is to act upon a metallic oxide with a solution of the acid. Acids, which, in such a process as this, can part with no less than the whole of their hydrogen, are called Monobasic adds, and can form only one salt. (Ex. Hydrochloric Acid, HCI; Acetic Acid, CH8 CO.OH.) Acids which can part with their hydrogen in two halves are capable of forming two salts, and are hence called Dibasic. (Ex. Sulphuric Acid; Oxalic Acid.) The definition of a Tribasic acid is precisely similar. (Ex. Phosphoric Acid; Citric Acid.) Acids are sometimes classified as inorganic and organic; the former are extremely powerful and corrosive, and do not, on this account, exist normally in nature. Sulphuric, nitric, and hydrochloric are the principal inorganic acids. Organic acids are produced by the activity of living tissue in plants and animals; they frequently occur in the free state. (Ex. Citric Acid in lemons.) Acetic, oxalic and tartaric are important acids of this class. Solubility in water, sourness, the power of reddening blue litmus paper, also of effervescing with alkaline carbonates, and of neutralising and being neutralised by alkalis; all these are characteristic properties of acids, although not necessarily essential.