and particularly the Oxidation of Iron. 233 



other circumstances are the same, the pnissiate is neutral ; 

 the one differs from the other only in the proportion of their 

 constituent principles ; the former contains an excess of ox- 

 ide, while in the latter the acid is merely saturated with it. 

 Thus the acids instantly render the prussiate of white iron 

 greenish, by taking from it a part of its base ; and the alkalis 

 make the slightly green prussiate of iron pass to a white co- 

 lour, by attracting to themselves a part of the acid which 

 enters into its composition : thus by art we may transform 

 these two salts, the one into the other at pleasure, merely 

 by varying the quantity of the substances which form them. 



These precipitates are not simple prussiates of iron, as has 

 been believed for a long time ; they contain besides prussiate 

 of potash, as Berthoilet has proved. The prussiate of pot- 

 ash has even so much affinity for the prussiate of iron, that 

 the sulphuric acid only decomposes in part the insoluble 

 combination which these two salts form together. If a 

 slight excess of acid only is added, the residue still contains 

 plenty of potash, and is perceptibly green ; it becomes 

 greenish blue by a greater quantity of acid, and, when ana- 

 lysed, little alkali and oxide is found. in it : this attraction is 

 sti!! more striking when the prussiate of potash is abundant 

 enough to render the combination soluble; then the sul- 

 phuric acid does not disengage from it even the most trifling 

 smell of bitter almonds, and the sulphate of iron uniformly 

 produces a very great quantity of precipitate; while, if the 

 alkaline salt is pure, tVie carbonic acid is powerful enough to 

 disengage the prussic acid from it. 



The same results are obtained by substituting, in place of 

 the acidulated sulphate of iron not much oxidated, the green 

 acidulated sulphate of iron and the red muriate of iron which 

 is only little acid : its precipitates are always triple prussiates 

 of iron and potash ; that one the oxide of which is green, is 

 less blue than the one the oxide of which is red ; botl^ 

 cliange colour if united with a great excess of oxide; the 

 first becomes yellowish white, and the second reddish yel- 

 low ; if the excess of oxide is less, they approach nearer 

 to a blue colour ; but both the one and the other are sus- 

 ceptible of being briehlened by the atids. 



