ALKALI, (Encycl.) Alkalies comprehend one class of the chemical elements, or component parts or neutral salts, as acids do another. Concerning the origin of alkalies there have been a great number of various conjectures, as well as concerning that of acids; but nothing certain has been, or in all probability will be, discovered. They are distinguished from other bodies of the saline kind by the following properties. 1. They combine with acids more readily than any other known substance, excepting the pure ponderous earth, occasioning an effervescence and heat. 2. When they are in a concrete form, into which they may be easily reduced by evaporation; if exposed to the open air, they attract the moisture from it, though not so strongly as concentrated acids; and generally deliquesce,
that is, become a fluid. But if they are mixed with water all at once, a considerable degree of heat is produced. 3. They generally change the blue and red colour of vegetables to a green; whereas the acids change it to red, or to a stronger red. 4. They have an acrid burning taste, which has something of urinous. 5. They partly exhale with water, especially when boiled in open vessels. 6. They may be fused by a moderate heat; and in that state they dissolve all sorts of earths, especially those called vitrifiables; and if the heat is sufficient, they and the earth form a brittle substance called glass; and upon this principle common glass is made.
Alkalies are principally distinguished into fixed and volatile; the former being not so easily dissipated even by a strong fire; for which reason they are commonly employed in making glass, and other compositions. But the volatile alkalies are so easily dissipated, that they may be almost entirely evaporated by a much less degree of heat than that of boiling water. Notwithstanding that various sorts of alkali are enumerated, it is supposed that there is but one alkaline principle in nature, which, by being variously combined with sundry substances, assumes various particular properties. However, an alkali is never found pure in nature, but it is always combined with other substances, from which it must be separated by art, in order to obtain it sufficiently pure. The fixed alkali is obtained either from sea-salt, or from vegetables; hence it is called fixed mineral alkali in the first case, and fixed vegetable alkali in the second. The volatile alkali is obtained, by means of decomposition and putrefaction, from all animal substances, and may be also obtained by decomposition and other means from some vegetable and a few other matters. Fixed alkali, whether vegetable or mineral, has always the same principal properties; yet they differ from one another in degree, viz. the mineral alkali possesses the alkaline properties in a less degree than the vegetable alkali; for when it is exposed to the open air, it will not attract the moisture from it in such a quantity as to deliquesce; when mixed with water, it will not occasion so much heat as the fixed vegetable alkali, &c.—Its affinity with other substances is exhibited below:
The fixed vegetable alkali has an affinity with the following substances:
Alkaline salts are of great use in chemistry, not only because they counteract the action of acids, but also because they are powerful solvents; and, when combined with other substances, communicate to them part of their saline properties. Thus, when they are com-
combined with sulphur, they make a compound called hepar sulphuris, or liver of sulphur, which is dissolvable in water, whereas sulphur by itself is not; and when they are combined with oils, form compounds called soaps, which are dissolvable in water, whereas oil by itself is not. The fixed alkali, more commonly used, by reason of its greatest purity, is that extracted from tartar, which is a substance found adhering to the sides of casks that have contained wine, and is considered as the essential salt of that liquor. This alkali, when it is in a liquid form by its combination with a sufficient quantity of water, is commonly, but improperly, called oil of tartar.
Volatile alkali, when first extracted from animal or vegetable substances, is very impure; but by proper methods it may be purified to a great degree; and in that purified state it differs from fixed alkali, principally because it is very volatile: it has a strong penetrating smell, which occasions coughs, suffocation, &c. — The hart's-horn drops commonly used, the smell of which almost every body knows, are a volatile alkali.