SULPHATES OF POTASH, SODA, AND LIME. 107 



without sensible action on animal or vegetable substances. It 

 consists, in 100 parts, of 



Crystallised. Dry, or waterfree. 



Sulphuric acid, . . 24-85 ... 56'18 



Soda, . . . 19-38 ... 43'82 



Water, . . . 55*77 



100 



100 



* 



Sulphate of soda is therefore much more soluble, as it is also 

 much lower in price, than the sulphate of potash. 



3. Sulphate of lime, (gypsum,) when pure, is colourless, and 

 without appreciable taste or smell. It usually occurs crystal- 

 lised, and often forms masses of crystals that possess a beauti- 

 ful satiny fracture and lustre. It undergoes no change by 

 exposure to the air; but when heated to about 300 F., it loses 

 21 per cent of water, and becomes white, opaque, and much 

 more friable, and easily reduced to an exceedingly fine powder. 

 After being thus burned, it forms what is called burned gypsum, 

 or plaster of Paris. When mixed quickly with water, and 

 poured into a mould, gypsum thus burned absorbs and combines 

 with the water which had previously been driven off by the 

 heating, and forms a somewhat solid sonorous mass, hence its 

 use for making plaster casts. Crystallised or native gypsum 

 dissolves in about 400 parts of water at 60, and burned gypsum 

 in about 500 parts. 



Sulphate of lime consists in the 100 parts of 



Native gypsum. Burned gypsum. 



Sulphuric acid, . . 46'31 ... 58-47 



Lime, . . . 32-90 ... 41-53 



Water, . . . 20'79 



100 

 100 



It is cheaper than either of the other sulphates, but much less 

 soluble in water. Its solution in water is very readily decom- 

 posed by the presence of organic matter in the water. It is 

 deprived of its oxygen by the organic matter, is converted into 

 sulphuret of calcium, and thus imparts to the water a sulphury 

 smell and taste. In making use of burned gypsum for'experi- 



