148 Progress of Foreign Science. 



Berzelius' view be correct, it is merely hydrocyanic acid which 

 should be liberated in this case ; whilst on the contrary, M. 

 Robiquet finds that it is the ferro-cyanic acid of Mr. Porrett. 

 M. Robiquet conceives, on mixing a solution of a neutral salt 

 of black oxide of iron with one of triple prussiate of potash, 

 in the fabrication of Prussian blue, that till the iron passes 

 to the maximutn of oxidizement, the precipitate always retains 

 a proportional quantity of alcali, which maintains the neu- 

 trality of the precipitate ; and, that the perfection of Prussian 

 blue consists chiefly in the complete peroxidation of the iron, 

 and in the total abstraction of the potash ; a result which can 

 be obtained only by the joint agency of oxygen and an acid. 

 Acting otherwise we do not obtain Prussian blue properly so 

 called ; so much so, that that of M. Berzelius formed with a 

 solution of black oxide of iron, and simple washing with distilled 

 water, did not acquire, notwithstanding its exposure to the 

 atmospheric air, the qualities of ordinary Prussian blue. " The 

 Prussian blue thus prepared," says Berzelius, " possesses 

 properties which it does not do, when it is otherwise prepared ; 

 it is soluble in pure water," &c. 



M. Robiquet hence thinks that M. Berzelius is mistaken in 

 regarding Prussian blue as a subsalt, or at least as containing 

 an excess of oxide. Besides, how can we admit that Prussian 

 blue contains an excess of oxide, since, when it is treated by 

 the most energetic and concentrated acids, we precipitate it 

 entirely, and without abstraction of any portion of oxide, by 

 the mere addition of distilled water. If after triturating Prus- 

 sian blue along with concentrated sulphuric acid, we leave it 

 to settle, and then decant the supernatant acid, we shall not 

 find in it any trace of iron. This is not, however, the case 

 with muriatic acid ; but here the residuum contains an excess 

 of ferro-cyanic acid, since it was by this process that M. 

 Robiquet succeeded in separating that acid from Prussian 

 blue. Nor are the results obtained by M. Berzelius incom- 

 patible with the existence of this acid. In fact, when it combines 

 with a base, the oxygen of the latter may be conceived to 

 unite with the hydrogen of the acid ; and the abstraction of 

 these two elements by evaporation, would leave a compound 

 represented by the constituents of cyanide of iron, and those 

 of the other base, so that the analysis of this combination 

 would present merely cyanogen and the metallic radicals. 

 This transformation of the ferro-cyanates into double cyanides, 

 does not take place undoubtedly for all the bases, and Prussian 

 blue is itself a striking example of it, since however it be 

 desiccated, it exhibits always hydrogenated products by its 

 calcination in close vessels. 



M. Robiquet agrees with M. Berzelius with regard to the 

 proportion of water, potash, and iron, in the triple prussiate ; 



