We must note the appearance of the term _vitriol_, applied to the solid salt-like bodies obtained from acids and characterized by a glassy lustre. By the middle of last century the vitriols were recognized as all derived from, or compounded of, sulphuric acid (oil of vitriol) and metals; this led to a subdivision of the large class of neutral salts into (1) metallic salts produced by the action of sulphuric acid on metals, and (2) neutral salts produced by the action of earths or alkalis on acids generally. To Rouelle, a predecessor of Lavoisier, who died four years before the discovery of oxygen, we owe many accurate and suggestive remarks and experiments bearing on the term "salt." I have already mentioned that it was he who applied the word "base" to the alkali or earth, or it might be metal, from which, by the action of acid, a salt is built up. He also ceased to speak of an acid as _sal acidum_, or of an alkali as _sal alkali_, and applied the term "salt" exclusively to those substances which are produced by the action of acids on bases. When the product of such an action was neutral--that is, had no sour taste, no soapy feeling to the touch, no action on vegetable colours, and no action on acids or bases--he called that product _a neutral salt_; when the product still exhibited some of the properties of acid, _e.g._ sourness of taste, he called it _an acid salt_; and when the product continued to exhibit some of the properties of alkali, _e.g._ turned vegetable reds to blue, he called it _an alkaline salt_. Rouelle also proved experimentally that an acid salt contains more acid--relatively to the same amount of base--than a neutral salt, and that an alkaline salt contains more base--relatively to the same amount of acid--than a neutral salt; and he proved that this excess of acid, or of base, is chemically united to the rest of the salt--is, in other words, an essential part of the salt, from which it cannot be removed without changing the properties of the whole. But we have not as yet got to know why certain qualities connoted by the term "acid" can be affirmed to belong to a group of bodies, why certain other, "alkaline," properties belong to another group, nor why a third group can be distinguished from both of these by the possession of properties which we sum up in the term "earthy." Surely there must be some peculiarity in the composition of these substances, common to all, by virtue of which all are acid. The atom of an acid is surely composed of certain elements which are never found in the atom of an alkali or an earth; or perhaps the difference lies in the number, rather than in the nature of the elements in the acid atoms, or even in the arrangement of the elementary atoms in the compound atom of acid, of alkali, and of earth.