PROTOCOMPOUNDS OF IRON. 559 



neously with alum, by the oxidation of iron pyrites, (page 

 516.) 



The commercial salt is in large rhomboidal crystals, from an 

 oblique rhomboidal prism, which effloresce slightly in dry air, 

 and when at all damp, absorb oxygen, and become of a rusty 

 red colour ; hence the origin of the French term couperose ap- 

 plied to this salt, and corrupted in our language into copperas. 

 If these crystals be crushed and deprived of all hygrometric 

 moisture by strong pressure between folds of cotton cloth or filter 

 paper, they may afterwards be preserved in a bottle without any 

 change from oxidation. Of the 7 HO which copperas contains, 

 it loses 6HO at 238, but retains 1 eq. even at 535. It can be 

 made, however, perfectly anhydrous, with proper caution, with- 

 out any appreciable loss of acid. It was observed by Mitscher- 

 lich to crystallize at I7(> j with 4HO, in a right rhombic prism, 

 like the corresponding sulphate of manganese. When its so- 

 lution, containing an excess of acid, is jevaporated by heat, a sa- 

 line crust is deposited, which, according to Kuhn, contains 

 3 HO. Another hydrate has also been obtained by dissolving 

 the sulphate in sulphuric acid, which contains 2HO, and has 

 the crystalline form and sparing solubility of gypsum (Mits- 

 cherlich). The sulphate of iron appears to form neither acid 

 nor basic salts. One part of copperas requires to dissolve it in 

 the following quantities of water, at the particular temperatures 

 indicated above each quantity, according to the observations of 

 Brandes and Firnhaber : 



50 59 75.2 109.40 114.80 140 183.2 194 212 

 1.641.43 0.87 0.66 0.440.380.370.270.30 



The sulphate of iron undergoes decomposition at a red heat, 

 changes into sulphate of the peroxide, and leaves, after all the 

 acid is expelled, the red peroxide known as colcothar. This 

 sulphate, like all the magnesian sulphates, forms a double salt 

 with sulphate of potash, containing 6 HO. A solution of the 

 sulphate of iron absorbs nitric oxide, and becomes quite black ; 

 it takes up the gas in the proportion of 9 parts to 100 anhy- 

 drous salt, according to Peligot, or one-fourth of an equivalent 

 (page 288). 



Protonitrate of iron may be formed by dissolving the 

 protosulphuret of iron in nitric acid, diluted and cold; the solu- 

 tion evaporated in vacuo gives pale green crystals, which are 



o o 



