ALKALI  , an Arabic

term originally applied to the ashes of plants, from which by lixiviation carbonate of soda was obtained in the case of sea-plants and carbonate of potash in that of land-plants . The method of making these " mild " alkalis into " caustic " alkalis by treatment with lime was practised in the time of Pliny in connexion with the manufacture of soap, and it was also known that the ashes of shore-plants yielded a hard soap and those of land-plants a soft one . But the two substances were generally confounded as " fixed alkali " (carbonate of ammonia being " volatile alkali "), till Duhamel du Monceau in 1736 established the fact that common salt and the ashes of sea-plants contain the same base as is found in natural deposits of soda salts (" mineral alkali "), and that this body is different from the " vegetable alkali " obtained by incinerating land-plants or wood (pot-ashes) . Later, Martin Heinrich Klaproth, finding vegetable alkali in certain minerals, such as leucite, proposed to distinguish it as potash, and .at the same time assigned to the mineral alkali the name natron, which survives in the symbol, Na, now used for sodium . The word alkali supplied the symbol for potassium, K (kalium) . In modern chemistry alkali is a general term used for compounds which have the property of neutralizing acids, and is applied more particularly to the highly soluble hydrates of sodium and potassium and of the three rarer "alkali metals," caesium, rubidium and lithium, also to aqueous ammonia . In a smaller degree these alkaline properties are shared by the less soluble hydrates of the "metals of the alkaline earths," calcium, barium and strontium, and by thallium hydrate . An alkali is distinguished from an acid or neutral substance by its action on litmus, turmeric and other indicators .