ACIDS. An acid is a chemical compound distinguished by the property of combin ing with bases in definite proportions to form salts (q.v.). The most striking charac teristics of A. are a sour taste, and the property of reddening vegetable blues. They are also mostly oxidized bodies; and at one time oxygen was thought to be essential to an acid, as the name oxygen (the acid-producer) indicates. Subsequent experience has extended the definition. There is an important class of undoubted A. that contain no oxygen; and silex or flint, which, being insoluble, neither tastes sour nor reddens litmus-paper, is held to be an acid because it combines with bases and forms com pounds like acknowledged A. The oxygen A., which are by far the most numerous class, are formed of elements (sulphur, nitrogen, chromium, etc., with two or more equiv alents of oxygen. The elements that form the strongest A. with oxygen are the non-metallic, and most of them have more than one stage of acid oxidation. Thus sul phur, with two equivalents of oxygen, forms sulphurous acid, symbol SO,; with three equivalents it forms sulphuric acid, symbol Similarly arsenic gives rise to arsenious acid (AsO,) and arsenic acid (As0,), The higher stage of oxidation forms the stronger and more stable acid. All metals, except arsenic, that form A. with oxygen, have also, at a lower stage of oxidation, one or more oxides. To these oxygen A. must be added
the organic A., composed either of carbon and oxygen, as oxalic acid or of these two along with hydrogen, as acetic acid (C,I1,0,) and formic acid (C,H0,). There are also A. found in animal fluids or resulting from their decomposition, which contain nitrogen in addition to the three elements above named; such is uric acid The hydrogen A. are formed of hydrogen and a radical, either simple or compound. The most important of these, and the type of its class, is hydrochloric or muriatic acid (C1H); others are hydriodic (III) and hydrocyanic A. (NC,H). As all A., however, even oxygen A., possess acid properties—i.e., combine with bases—only when in combination with water, a new view of the constitution of A. is beginning to prevail, which makes hydrogen the real acidifying element in all A. Thus, instead of considering vitriol as a compound of sulphuric acid and water (S0,-1-110), the hydrated acid is held to be the real sulphuric acid, and its rational formula to be (S0,11). It thus becomes analogous to hydrochloric acid (CHI). This view has not only the advantage of bringing all A. into one class, but makes the theory of their combination with bases and of their capacity of saturation uniform and simple. See CHEMISTRY.