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Common gypsum or selenite, such as that found at 
Shotover hill near Oxford, contains, besides sul- 
phuric acid and lime, a considerable quantity of 
water; and its composition maybe thus expressed: 
Sulphuric acid, one proportion 7 5 
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 deficient. Gypsum free from water is 
sometimes found in nature, when it is called anhy- 
drous 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 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 500 times its weight of cold water, 
and is more soluble in hot water ; so that when 
water has been boiled in contact with gypsum, 
crystals of this substance are deposited as the water 
cools. Gypsum is easily distinguished when dis- 
solved by its properties of affording precipitates to 
solutions of oxalates and of barytic salts. 
Great difference of opinion has prevailed amongst 
agriculturists with respect to the uses of gypsum. 
