162 Dr. Williamson on the 



This quantity of potassium is too small to justify the con- 

 clusion that it is essential to the composition of the salt, as it 

 only amounts to about 1 equiv. to 24 iron. Its equivalent of 

 iron was therefore added to the other iron, and in this man- 

 ner numbers found which correspond to the formula Cy 7 Fe 5 

 -f 5 aq, as is shown by the following : — 



Calculated. 

 C 14 84 = 23-14 

 Fe 5 136 = 37-5 

 Aq 5 45 = 12-4 



On treating this body with potash, peroxide of iron is se- 

 parated and a brownish-red liquid formed, in which proto- 

 and persalts of iron give a blue precipitate. This liquid is 

 decomposed by boiling, peroxide of iron is precipitated, and 

 the colour of the precipitate becomes much lighter. I am 

 not aware, however, what is the nature of the decomposition. 

 The brown-red colour of the liquid has much similarity with 

 that obtained by mixing ferridcyanide of potassium with a 

 persalt of iron. The green substance which Pelouze obtained 

 by decomposing yellow prussiate by chlorine, and for which 

 he gives the empirical formula Fe 3 Cy 4 , is decomposed in a 

 similar manner by potash. If, in accordance with the view 

 of many chemists, we consider this substance analogous to 

 magnetic iron ore, as Fe 2 Cy 3 + Fe Cy, the compound which 

 I have described may be viewed as containing a double quan- 

 tity of percyanide, 2(Fe 2 Cy 3 ) + Fe Cy. I will not however 

 venture to express any opinion as to the propriety of consi- 

 dering it as a peculiar compound or not. 



It is well known that for technical purposes an esteemed 

 blue colour, which is sometimes called TurnbulPs blue, is 

 prepared by decomposing ferridcyanide of potassium with a 

 protosalt of iron. In this compound, which by its colour 

 may be easily distinguished from prussian blue, the presence 

 of potassium has been discovered, but without bringing to 

 light any connection with the composition of the body. It 

 appeared to me not devoid of interest to examine the pro- 

 perties of this substance and subject it to analysis. To a 

 dilute aqueous solution of ferridcyanide of potassium, which 

 had been purified by frequent crystallizations, was added 

 a solution of sulphate of iron, the red salt remaining in ex- 

 cess. In washing out this precipitate T proceeded with par- 

 ticular care, as, whilst removing all substances mixed with 

 the compound, it was necessary to avoid any action upon 

 it which might render soluble any constituents of the salt, 

 as was the case with Berzelius's washing in the air. This 

 object I completely succeeded in attaining, simply by means 



