1316. The nature of gypsum is easily demonstrated: if oil of vitriol be added to quicklime, there is a violent heat produced; when the mixture is ignited, water is given off, and gypsum alone is the result, if the acid has been used in sufficient quantity; and gypsum mixed with quicklime, if the quantity has been deficient. Gypsum, free from water, is sometimes found in nature, when it is called anhydrous selenite; it is distinguished from common gypsum by giving off no water when heated. When gypsum, free from water, or deprived of water by heat, is made into a paste with water, it rapidly sets by combining with that fluid. Plaster of Paris is powdered dry gypsum, and its property as a cement, and its use in making casts, depend upon its solidifying a certain quantity of water, and making with it a coherent mass. Gypsum is soluble in about 500 times its weight of cold water, and is a little more soluble in hot water. It is commonly found in spring water, which in part owes its hardness to the presence of this salt. Gypsum is easily distinguished by its properties of affording precipitates to solutions of oxalates and of barytic salts. It has been much used in America, where it was first introduced by Franklin on his return from Paris, where he had been much struck with its effects. He sowed the words, This has been sawn with gypsum, on a field of lucern, near Washington; the effects astonished every passenger, and the use of the manure quickly became general, and signally efficacious. It has been advantageously used in Kent, but in most counties of England it has failed, though tried in various ways, and upon different crops.