158 Dr. Williamson on the 



the purpose of gaining some further insight into the compo- 

 sition and nature of these salts, and of deciding the question 

 of the existence of the pure compounds of iron of which the 

 formulae have been given on purely theoretical grounds. 



It is well known that when ferrocyanide of potassium is 

 boiled with dilute sulphuric acid, only a part of the cyanogen 

 is given off as hydrocyanic acid, and that a greenish substance 

 remains behind containing all the iron and a considerable 

 quantity of the potassium in combination with cyanogen ; also 

 that this substance by exposure to the air gradually changes 

 into a deep blue. This process is much favoured, as Gay- 

 Lussac observed, by the presence of free acid, and is accom- 

 panied by the formation of a potassium salt of this acid. This 

 green substance consists, according to the analysis of Everitt, 

 which M. Geiger repeated in this laboratory, of 3 equivalents 

 cyanogen, 2 iron and 1 potassium. My analysis confirms this 

 composition. 2*372 grammes ferrocyanide of potassium were 

 distilled with water, and sulphuric acid equivalent to the quan- 

 tity of potassium in the salt, and the distillation was continued 

 until no more prussic acid passed over. The residue, after 

 this treatment, was washed out on a filter, air being excluded, 

 dried and cautiously heated in a platinum crucible with con- 

 centrated sulphuric acid, which decomposed all the cyanogen 

 and converted the iron and potassium into sulphates. The 

 aqueous solution of these salts was decomposed by ammonia, 

 filtered, evaporated and heated to redness. 0*532 gramme 

 sulphate of potash was thus obtained, which corresponds to 

 10*08 per cent, of potassium instead of 10*67 P er cent., 

 which the above formula requires. 



The compound may be considered as ferrocyanide of potas- 

 sium, in which 1 atom of potassium is replaced by iron, and 

 may be expressed by the following formula : — 



n<* fFe 



Its reactions are also such as we must expect from such a 

 compound, for by treating it with caustic potash, a solution of 

 pure ferrocyanide of potassium is formed whilst protoxide of 

 iron is set free. It will be designated in the following lines, 

 ferrocyanide of iron and potassium. The blue compound into 

 which this salt is changed by oxidation has been incorrectly 

 considered as prussian blue. It, however, differs materially 

 in its composition from that body, as I will proceed to show. Its 

 formation takes place very easily, for it is caused by every re- 

 agent, such as chlorine, nitric acid, sulphuric acid, with oxy- 

 gen, &c, which eliminates potash from the white salt. For 

 its preparation, dilute nitric acid, which consists of 1 volume 

 concentrated acid and 20 of water, may be used with most 



