340 Materials for a History of the Prussiates. 



Prussian blue is not of a nature to be combined without a 

 residue with potash. In a word, the triple prussiate, with 

 the exception of- its alkaline base, is, if I may so express 

 myself, a compound as to which we have as yet no data, 

 and no idea which authorizes us to consider it rather as a 

 salt, the acid of which would have been particularly exalted 

 by its union with the oxide, than as a combination perfected 

 throughout all its parts by this same oxide. 



One property which seems to hinder us from admitting 

 the prussiate as a salt, the acid of which should be exclu- 

 sively united to the black oxide, is that of its resisting the 

 power of the alkaline hydro-sulphurets. If these re-agents, 

 which spare no other metallic salt, have no action upon the 

 triple prussiate, we are therefore to a certain extent war- 

 ranted in presuming that the oxide of iron could not 1 well 

 have been exclusively attached to the acid of the triple prus- 

 siate, unless we are willing to believe that the affinity of 

 this acid for the oxide is not powerful enough to defend it 

 from the fate which is common to all the other oxides. To 

 conclude: We shall see presently that an affinity so extra- 

 ordinary, however unexampled it may be in chemistry, is 

 not an impossibility. I shall now proceed to the trial of the 

 hydro-sulphuret of potash upon the triple prussiate. 



Hydro-sulphuret and triple Phosphate. — The hydro-sul- 

 phuret of potash or of ammonia, even when assisted by heat, 

 has no action upon this salt. If it contained some remains 

 of ferruginous carbonate, it would be freed from it, because 

 the hydro-sulphuret decomposes the latter : we may filter it 

 if necessary, and yet the prussiate still crystallizes under the 

 accustomed form. A similar result leads us to discover, as 

 we originally insinuated, a most particularly intimate com- 

 bination between the three elements of the triple prussiate. 

 But we shall see these same hydro-sulphurets contribute in 

 enabling us to obtain, in all its purity, the white prussiate, 

 or that union in which the iron is at its minimum of oxida- 

 tion, which I proved in my first memoir upon Prussian blue. 



While Prussiate. — On the one hand we must have a flask 

 of green sulphate very much diluted, at the bottom of 

 which we keep some grains of sulphuret of the same metal, 



in 



