ON PRUSSIAN BLUE 241 



it is therefore a triple salt. Now. when an acid is added 

 to this salt, the colouring matter, in consequence of the 

 stronger attraction of acids for alkalies, must be expelled, 

 which immediately, on account of its volatility, goes over 

 into the receiver during the distillation. But as the acids 

 are not capable of expelling this colouring matter from the 

 iron, the calx of iron will contain as much as is requisite 

 for its saturation, or, what comes to the same, as much as 

 is requisite for the production of Prussian blue ; and this 

 is the part which separates from the salt during the distilla- 

 tion. If Berlin blue be extracted by lime or terra .ponderosa, 

 these extracts show the same phenomena during distillation 

 with vitriolic acid. 



SECTION VII. 



If Prussian blue be extracted by volatile alkali, a com- 

 pound arises, which likewise constitutes a kind of triple 

 neutral salt, consisting of volatile alkali, iron, and the 

 colouring matter. With vitriolic acid it shows the same 

 phenomena as the salt of Sec. vi. If this neutral salt be 

 distilled by itself, after having been dissolved in water, the 

 solution grows thick, in consequence of the separation of 

 Berlin blue, and a- volatile alkaline liquor goes over into the 

 receiver. If the distillation be continued till but little 

 liquid remains with the Berlin blue in the retort, no more 

 salt will be found in the retort, but all will have gone over 

 into the receiver. The liquor in the receiver consists of 

 volatile alkali and the colouring matter. It is not pre- 

 cipitated by lime-water ; but vitriolated iron is decomposed 

 by it, and, on adding an acid, Prussian blue is generated. 

 If a piece of paper dipped into a solution of vitriolated iron 

 be suspended in the receiver, this solution is soon decom- 

 posed, the air of the receiver being impregnated with volatile 

 16 



