344 IRON, NICKEL, COBALT, &c. 



tured for the use of dyers and other artists. It 

 crystallizes in four-sided oblique prisms with a 

 rhombic base, is transparent, has a light green 

 colour, and a sweetish astringent taste. It may 

 be obtained perfectly pure from the copperas 

 manufactories ; or by dissolving the salt of com- 

 merce in boiling water, filtering the solution 

 while hot, and allowing it to crystallize by cool- 

 ing. I employed this salt to determine the 

 atomic weight of protoxide of iron, and I shall 

 now state the method by which I proceeded. 



1. 17'375 grains of pure dry crystals of proto- 

 sulphate of iron were dissolved in water, and 

 mixed with a solution of 13*25 grains of chloride 

 of barium. A double decomposition immediate- 

 ly took place, and sulphate of barytes precipi- 

 tated to the bottom of the vessel. When the 

 liquid had become clear after the subsidence of 

 this sulphate, it was tested by means of sulphate 

 of soda, and muriate of barytes ; but was not 

 affected by either of these reagents, showing that 

 it neither contained barytes nor sulphuric acid. 

 From this experiment it is evident, that the 

 barytes from 13*25 grains of chloride of barium, 

 just saturates the sulphuric acid in 17*375 grains 

 of crystallized protosulphate of iron. Conse- 

 quently, 17*375 grains of protosulphate of iron 

 contain exactly 5 grains of sulphuric acid. 



2. 17*375 grains of crystallized protosulphate 

 of iron were exposed for some time in a plati- 



