330

The nature of gypsum is easily demonstrated;if oil of vitriol be added to quicklime, there is aviolent heat produced; when the mixture is ig-nited, water is given off, and gypsum alone is theresult, if the acid has been used in sufficient quan-tity ; and gypsum mixed with quicklime, if thequantity has been deficient. Gypsum, free fromwater, is sometimes found in nature, when it iscalled anhydrous selenite. It is distinguished fromcommon gypsum by giving off' no water whenheated.

When gypsum, free from water, or deprived ofwater by heat, is made into a paste with water, itrapidly sets by combining with that fluid. Plasterof Paris is powdered dry gypsum, and its property asa cement, and in its use in making casts, dependsupon its solidifying a certain quantity of water, andmaking with it a coherent mass. Gypsum is solu-ble in about 500 times its weight of cold water, andis more soluble in hot water; so that when waterhas been boiled in contact with gypsum, crystalsof this substance are deposited as the water cools.Gypsum is easily distinguished by its properties ofaffording precipitates to solutions of oxalates and ofbarytic salts.

Great difference of opinion has prevailed amongstagriculturists with respect to the uses of gypsum.It has been advantageously used in Kent, and vari-ous testimonies in favour of its efficacy have beenlaid before the Board of Agriculture by Mr. Smith.

5