Materials for a History of the Prussiates. 51 



order to convert it into a triple salt, and may afterwards 

 furnish blue with the red solutions. In this way practice had 

 preceded theory in accomplishing an object; but practice 

 also becomes in its turn a rational formula, from the moment 

 that it is confirmed by theory. Two other experiments 

 strengthen this demonstration. 



The lixiviums are generally precipitated with a solution of 

 four parts of alum, and one of the sulphate of commerce. 



I divided one of these solutions into two parts : the one 

 was hyper-oxidated by the oxy-muriatic acid, and the other 

 not. I afterwards saturated them with carbonaceous lixi- 

 vium. The common solution furnished blue in abundance, 

 but the hyper-oxidated gave a very pale precipitate, which 

 was only a little blue mixed with a great deal of alumine. 

 This experiment does not differ essentially from the pre- 

 ceding. It has only the advantage of showing that the alum 

 is but a passive ingredient in the formation of Prussian blue. 



It is not the same therefore with the lixiviums of the ma- 

 nufacturer, as it is with an alkali passed over Prussian blue: 

 the latter will always give blue in abundance, because it 

 comes out of the operation tripled, but the lixiviums do not-. 

 They could not give it but in proportion to the triple prus- 

 siate which they contain : it is in order to increase it, or to 

 raise their simple prussiate to the same degree, that it is in- 

 dispensable to use, if not a sulphate rigorously green, at 

 least one which contains a certain quantity of green ; and 

 this is preciselv the usual quality of that of commerce, how- 

 ever old it may be. 



These details also explain to us, that if the lixiviums 

 contain a portion only of tripled prussiate, it is because 

 charcoal of blood has not iron enough to furnish for rais- 

 ing all the simple prussiate formed during calcination to 

 the triple degree, or rather because a part of the latter again 

 becomes simple prussiate by the loss of its oxide, is we have 

 seen happen to it when heated alone. Of these two opinions, 

 however, I adhere to the last, because I have remarked, 

 that the charcoal which served the lixiviums gives ashes 

 which always contain much iron : thus, in the calcination of 

 the alkaline- carbonaceous mixtures, we cannot presume with 



D 2 reason, 



