and part'icaiarly the Oxidation of Iron. 227 



m commerce. Nevertheless, besides this combination, the 

 white oxide may still form a sulphate much more acid ; and 

 then, instead of being of a dark green, the compound ap- 

 proaches to a clear emerald green. Thus we find there is a 

 green oxide as well as a red oxide of iron ; each of them, on 

 being united with sulphuric acid, gives birth to at least two 

 very distinct salts. Let us now proceed to examine these six 

 kinds of sulphate of iron. 



I shall distinguish the first of them by the liames of aci- 

 dulated sulphate and acid sulphate of white iron, because 

 the oxide in these is colourless, and the one much more acid 

 than the other ; the second, for the same reason, under the 

 names of acidulated sulphate and acid sulphate of green ironj 

 and the third, for the same reason, I shall term neutral and 

 acid sulphates of red iron. 



The acidulated sulphate of white irdn is obtained by boil- 

 ino- sulphuric acid diluted with water upon an excess of iron 

 filino-s or turnings. When dissolved, or in particular cry- 

 stallized, it is always of a bottle green, and the more co- 

 loured it is, the more is it esteemed in commerce. It in- 

 stantly loses this colour upon an addition of sulphuric acid j 

 it then takes an emerald green and becomes acid sulphate 

 of white iron, which alters bine colours much more than the 

 former* : it is then less fit for the operations of the arts, and 

 rejected by manufacturers, who give a preference to the for- 

 mer, not from prejudice, but founded on the nature of saline 



* When sulphuric acid is poured into a solution of acidulated sulphate of 

 white iron, evaporated in such a manner as to mark 36° to the weight of the 

 liquor, it almost immediately forms an abundant white and crystalline pre- 

 cipitate, which is nothing else but acid sulphate of white iron. This is thS 

 reason v/hy, in vitflol manufactories, it sometimes happens that the liquor be- 

 comes all at once troubled ta a certain degree, and leaves a deposit of a white 

 matter, which is rejected by the manufacturer, and gets the name of magne- 

 iia. The solution is then too acid, and the escape of this excess of acid takes 

 place instantly, so as to form an acid sulphate which is precipitated, and an 

 acidulated sulpliate which remains in solution and crystallizes much more re- 

 gularly than the first. This inconvenience might be guarded against by boil- 

 ing the solution a longer time with iron, and adding water to it, if necessary; 

 because the acidulated sulphate of white iron or the sulpliate of iron of com- 

 . merce, when it is too much evaporated, is also susceptible of partly concreting 

 itself; at least it sometimes docs so suddenly, all in a rauDS. 



p 2 compositions J 



