Earths thus came to be included in the term "alkali," when that term was used in its widest acceptation. But a little later it was found that some of the earths were thrown down in the solid form from their solutions in acids by the addition of alkalis; this led to a threefold division, thus-- Earths <----> Alkaline earths <----> Alkalis Insoluble Somewhat soluble Very soluble in in water. in water. water. The distinction at first drawn between "earth" and "alkali" was too absolute; the intermediate group of "alkaline earths" served to bridge over the gap between the extreme groups. "In Nature," says Wordsworth, "everything is distinct, but nothing defined into absolute independent singleness." At this stage of advance, then, an earth is regarded as differing from an alkali in being insoluble, or nearly insoluble in water; in not being soapy to the touch, and not turning vegetable reds to blue: but as resembling an alkali, in that it combines with and neutralizes an acid; and the product of this neutralization, whether accomplished by an alkali or by an earth, is called a salt. To the earth or alkali, as being the foundation on which the salt is built, by the addition of acid, the name of _base_ was given by Rouelle in 1744. But running through every conception which was formed of these substances--acid, alkali, earth, salt--we find a tendency, sometimes forcibly marked, sometimes feebly indicated, but always present, to consider salt as a term of much wider acceptation than any of the others. An acid and an alkali, or an acid and an earth, combine to form a salt; but the salt could not have been thus produced unless the acid, the alkali and the earth had contained in themselves some properties which, when combined, form the properties of the salt. The acid, the alkali, the earth, each is, in a sense, a salt. The perfect salt is produced by the coalescence of the saltness of the acid with the saltness of the alkali. This conception finds full utterance in the names, once in common use, of _sal acidum_ for acid, _sal alkali_ for alkali, and _sal salsum_ or _sal neutrum_ for salt. All are salts; at one extreme comes that salt which is marked by properties called acid properties, at the other extreme comes the salt distinguished by alkaline properties, and between these, and formed by the union of these, comes the middle or neutral salt. It is thus that the nomenclature of chemistry marks the advances made in the science. "What's in a name?" To the historical student of science, almost everything. We shall find how different is the meaning attached in modern chemistry to these terms, _acid salt_, _alkaline salt_, _neutral salt_, from that which our predecessors gave to their _sal acidum_, _sal alkali_, and _sal neutrum_.