Alkali (Arabic, al-qali, the ashes of the plant glasswort, yielding soda), the general name of a class of substances, such as cassia, rubidia, potash, soda, lithia, and ammonia, whose distinguishing peculiarities are solubility in alcohol and water, uniting with oils and fats to form soap, neutralizing and forming salts with acids, reddening several vegetable yellows, and changing reddened litmus to blue. These properties are the reverse of those of acids, and the two classes are regarded as antagonistic to each other. Some other substances, as lime, baryta, strontia, and magnesia, possessing some of the qualities of the alkalies as neutralizing acids, and changing the vegetable colors, are called alkaline earths. Pure anhydrous alkalies are exceedingly caustic, destroying vegetable and animal tissues. They abstract moisture rapidly on exposure to the air. Combined with carbonic acid and water, forming carbonates, they are used in medicine as diuretics and for correcting acidity, as well as for other effects. The alkalies and the earths also were until the present century regarded as simple substances. Lavoisier first suggested that they were metallic oxides.
Sir Humphry Davy proved this in 1807, by separating the metals, to which he gave the names potassium, sodium, barium, strontium, and calcium, the last the metallic base of lime. The discovery of these metals led to that of pure potash and soda. The alkalies were known before only in the state of hydrates, though incorrectly regarded as anhydrous.