Materials for a History of the /Prm slates, 34? 



is a point which also deserves some attention, when we see 

 that this same oxide can follow ihe prussic acid from one 

 combination to the other, without changing its state : when 

 we see also this oxide pass from prnssiate to prussiate, re- 

 turn from the latter to the former, circulate through even 

 the most oxidizing medium, without thereby losing the 

 state which constitutes it an oxide at the minimum : this is 

 also a point in the history of prussiates, which in my opinion 

 has not been attended to. 



If, for example, it would be correct to say, that without 

 the assistance of the black oxide the prussiate of potash 

 would neither be yellow, crystallizable, nor dyeing, we 

 might assert with equal truth, that the Prussian blue could 

 not be formed without the intervention of this same oxide • 

 and in fact when, with a solution of red oxide of triple prus- 

 siate of potash, we make Prussian blue, the black oxide ,ot 

 this salt passes jointly with its acid into the new combina- 

 tion : whence it follows that this' oxide, element of the prus- 

 siates of potash, becomes so afterwards from the Prussian 

 blue, and even as we shall show from all the other metallic 

 prussiates which are formed with this salt. 



This black oxide is found so solidly interwoven in the 

 combination of Prussian blue, so well guarded by its al- 

 liance with the prussic acid from all ulterior hyper-oxida- 

 tion, that we never fail to recover it in^his blue, such as it 

 was formerly in the triple prussiate of potash. T shall go 

 further : if we make blue with this prussiate and the green 

 sulphate, the oxide of this last will rise, as we know, to its 

 maximum, in proportion as the blue will be coloured by the 

 impression of the air ; but will it be the same with the black 

 oxide which passes jointly with the acid in Prussian blue ? 

 Certainly not. This oxide will not renounce the quality of 

 minor oxide which it had in the prussiate of potash : i. e. if 

 during the exposure to the air the base of the green sul- 

 phate, and consequently that of the white prussiate, is raised 

 from 28 to 48 per cent., the black oxide, the inseparable 

 attendant upon the prussic acid, will not participate in this 

 hvper-oxidation, it will invariably keep to its 28 per cent. 

 Not only the atmosphere, which so easily raises to their 



maximum 



