Acids. An acid in chemistry is any electro-negative compound, capable of combining in definite proportions with bases to form salts. Most of the liquid acids possess a sour taste, and redden litmus paper. The acids have been variously classed by different writers, as into organic and inorganic; metallic and non-metallic; oxygen acids, hydrogen acids, and acids destitute of either of these elements; the names being applied according to the kingdom of nature, or class of bodies to which the radical belonged, or after the element which was presumed to be the acidifying principle. Acids are in various forms; some are gaseous, as carbonic acid; some are liquid, as nitric and acetic acid; others are solid, as citric and oxalic acid; others again under peculiar conditions assume more than one of these forms. Acids, which are soluble or liquid, are corrosive, and more or less poisonous when concentrated. They change vegetable blues to red and neutralize the effects of alkalies on vegetable blues and yellows. Most of the acids are soluble in water in all proportions; they neutralize the alkalies, effervesce with the carbonates, and combine with the bases generally, forming compounds called salts. The methods for estimating the strength or neutralizing power of acids, as well as the strength of their solutions, will be found under Acidimetry, No. 78. The names of the acids end either in -ic or -ous; the former being given to that containing the larger portion of the electro-negative element, or oxygen, and the latter to that containing the smaller quantity.' As sulphuric acid, an acid of sulphur, containing 3 atoms of oxygen; sulphurous acid, another sulphur acid, containing only 2 atoms of oxygen. "When a base forms more than 2 acid compounds with oxygen, the Greek preposition hypo is added to that containing the smaller portion, as hyposulphuric and hypo-sulphurous acids. The prepositions per, hyper, and the syllable oxy are also prefixed to the names of acids when it is intended to denote an increase of oxygen, as hypernitrous acid, perchloric acid, oxymuriatic acid, etc.. The prefix hydro to the name of an acid denotes that the acid combination is with hydrogen, and not with oxygen. All the strong liquid acids should be kept in glass bottles, furnished with perfectly tight ground-glass stoppers; glass vessels should bo used in measuring them, and they should bo dispensed in stoppered vials. Fluoric acid must bo kept in a bottle made of lead, silver, platinum, or pure gutta-percha, as it acts readily on glass. In the combination of acids with bases to form salts, distinctive terminations are employed to denote the kind of acid present. The name of a salt of an acid ending in -ic, terminates in -ate ; thus, sulphate of soda, formed from sulphuric acid and soda. The name of a salt of an acid ending in -ous, terminates in -ite ; as sulphite of lime, formed from sulphurous acid and lime. The names of compounds formed by the union of non-metallic elements, and certain other bodies, with the metals or with each other, terminate in -ide or -urct; thus, sulphide or sulphuret of silver, formed of silver and sulphur. (Cooley.) In accordance with the scope of this work it has been found advisable to omit a number of acids, both simple and compound, of limited practical use ; the selection being confined to acids of more general utility and adaptation to practical purposes.