HISTORY OF PRUS3IATES. 99 



mcnt when the ternary combination begins to dissolve, con- 

 firms by its whiteness the fact, that it is only the oxide at a The black 

 minimum, which has the privilege of entering into the for- °||J deof ir0Ii 

 mation of the triple prussiate. This is one of those truths, triple prussiate.. 

 on which Scheele left nothing to be desired ; yet the dis- 

 tinction of the oxides in this substance is a point, to which 

 subsequent chemists have not paid all the attention it 

 deserves. 



Black Oxide, mi Element of Prussian Blue. 



We have just shown, that this oxido, in a constant pro- it is one es- 



portion, is essential to the constitution of the triple prus- semiaI I macl P 3ei 



of prussian '- 

 siate; but there is another object, that has also some claim blue. 



to attention, which is, that this oxide is capable of follow- 

 ing the prussic acid from one combination to another, with- 

 out changing its state ; that it can pass from prussiate to 

 prussiate and back again, and even circulate through the 

 most oxiding mediums, without losing the state of a mini- 

 mum oxide: and this I conceive to be a point of view, 

 which has been overlooked in the history of prussiates. 



If, for instance, we may say with truth, that the prus- 

 siate of potash would be neither yellow, nor crystallizable, 

 nortingeing; we may assert with equal foundation, that 

 neither would prussian blue be formed without the inter- 

 vention of this^oxide : and in fact, when we make prussian 

 blue with a solution of red oxide and of triple prussiate 

 of potash, the black oxide in the latter salt enters into the 

 new combination jointly with its acid; whence it follows, 

 that this oxide, which is an element of the triple prussiate 

 of potash, becomes so afterward of prussian blue; and 

 even, as will be seen presently, of all the other metallic 

 prussiates, that are made with this salt. 



This black oxide is so firmly intermixed in the compound Resists farther 

 of prussian blue, and so well defended from all farther wn en thS 

 oxigenation by its union with the prussic acid, that we combined. 

 never fail to find it again in this blue such as it was in the 

 triple prussiate of potash. I will say more; if we make 

 the blue with this prussiate and the green sulphate of iron, 

 the oxide of the latter will be raised, as is well known, to 

 its maximum, in proportion's the blue becomes coloured 



by 



