SULPHATES. 305 



SULPHATES. 



768. The sulphates, with the ex- 



What is said 



of the color ception of those of the alkaline 



are f r the mOSt 



ble salts, of colors corresponding to 

 the solutions of the corresponding metals. The 

 figure represents a crystal of gypsum. The form 

 longs to the fourth system. 



769. PREPARATION. The sulphates are 



How are the .,,,., , , ,. 



sulphates produced either by the direct combination 

 of sulphuric acid with the proper oxide, or 

 by its action on the metals. The latter has been already 

 particularly described in the section on sulphuric acid. 

 They are also sometimes formed in nature, by the 

 action of the air on sulphurets. In this action, the 

 metal is converted into oxide, and the sulphur into acid, 

 which together form the sulphate. Green vitriol is 

 sometimes thus formed in soils, from sulphuret of iron 

 ox fool's gold. 



What ix gyp- ?70. SULPHATE OF LIME. - GYPSUM. - 



sum? This is a white, soft mineral, occurring 



abundantly in nature. The finer kinds are known as 

 alabaster. Finely ground, it is employed extensively 

 as a fertilizer of the soil under the name of plaster. 

 Plaster of Paris is produced by heating gypsum until 

 its water is expelled. The plaster, when pulverized, 

 has the property of setting with water, or, in other 

 words, forming a hard coherent mass. 



771. PLASTER CASTS. These are pro- 



How are plas- . 



ter casts pro- duced by reducing the burned or powdered 

 duced? gypsum to the consistence of cream, with 



