NITRATES. 309 



common name of vitriols. Sulphates of zinc, copper, 

 and iron are called respectively white, blue, and green 

 vitriol. Green vitriol readily absorbs oxygen from the 

 air, and becomes brown, from the accumula- 

 tion of peroxide of iron upon its surface. A 

 solution of it is changed to a yellowish-red 

 color, by the oxidizing action of either nitric 

 acid or chlorine. A crystal of blue vitriol is 

 represented in the figure. The form belongs to the 

 fifth system. 



NITRATES. 



How are ni- 781. The nitrates are formed by 

 transformed? fa^ act ion of nitric acid on metals, 

 as already explained, and also by the action of 

 the acid on oxides previously formed. In the 

 latter case, the metallic oxide takes the place of 

 the water of hydration, which always belongs 

 to the acid. They are also produced by double 

 decomposition. The figure represents a crystal of salt- 

 petre. The form belongs to third system. This latter 

 method is illustrated below, in the preparation of nitrate 

 of potassa from the nitrate of lime. 



782. NITRATE OF LIME. This salt is 



How is nitrate . 



of lime, pro- of considerable interest, from the fact that 

 it is employed in the production of salt- 

 petre or nitre. It is formed in the so called, nitre beds, 

 by mixing together refuse animal matter with earth and 

 lime. In the gradual putrefaction of the animal mat- 

 ter which follows, its nitrogen takes oxygen from the 



