Page:The chemistry of paints and painting.djvu/53

Rh burnt lime, or quicklime, and slaked lime. The first of these is neutral and nearly insoluble in pure water, the second and third are alkaline and caustic. When burnt lime unites with water to form slaked lime it becomes slightly soluble in pure water. In chemical language these three compounds are called respectively calcium carbonate, calcium oxide, calcium hydrate (or hydroxide). From the first substance the others are readily obtained. If calcium carbonate, often called carbonate of lime, be heated to a sufficient temperature, it is decomposed, being resolved into carbon dioxide (carbonic acid gas) which escapes, and calcium oxide (lime) which remains: from 100 parts by weight of the carbonate 56 parts of lime, that is, burnt lime, are obtained. Placed in water or exposed to moist air this burnt lime combines with water, 56 parts of it uniting with 18 parts of water to yield 74 parts of slaked lime, calcium hydrate. In the ordinary country atmosphere, which contains no more than 3 measures of carbonic acid gas per 10,000, slaked lime or calcium hydrate loses its combined water, slowly becoming once more the carbonate from which it was originally produced: 74 parts of hydrate lose 18 parts of water and combine with 44 parts of carbonic acid, and yield 100 parts of carbonate. Thus mild lime is formed once more from caustic lime. By this change, if it be effected in the presence of a sufficiency of free water—that is, if the hydrate of lime be in the state of a firm paste—the whole substance becomes a hard crystalline solid, like an opaque marble. Advantage may be taken of this hardening or cementing process to firmly incorporate other substances with the lime, Silicious sand, infusorial earth, pumice, marble powder, and many other mineral substances, may be thus introduced. Such of these materials as are silicious may