L 



stance consists of sulphuric acid (the sains body that 

 exists combined with water in oil of vitriol) and lime; 

 and when dry it is composed of 55 parts of lime and 75 

 parts of sulphuric acid. Common gypsum or selenite, 

 such as that found at Shotover hill near Oxford, con- 

 tains besides sulphuric acid and lime, a considerable 

 quantity of water ; and its composition may be thus 

 expressed : 



Sulphuric acid one proportion 75 

 Lime one proportion - 55 



Water two proportions - 34 



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 defi- 

 cient. 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 ra- 

 pidly sets by combining with that fluid. Plaister of 

 Paris is powdered dry gypsum; and its property as a 

 cement, and in its use in making casts depends upon 

 its solidifying a certain quantity of water, and making 

 with it a coherent mass. Gypsum is soluble in about 

 50O times its weight of cold water, and is more solu- 

 ble in hot water ; so that when water has been boiled 

 in contact with gypsum, crystals of this substance are 



