168 Dr. Williamson on the 



as was remarked above in the case of another compound. 

 With other cyanogen compounds a similar action occurs, of 

 which I will mention a striking instance further on. On de- 

 composing this substance by potash, ferrocyanide of potas- 

 sium is formed and peroxide of iron set free. Experiments, 

 in which I endeavoured to determine quantitatively the pro- 

 portion of iron separated by potash, invariably gave more than 

 the formula of the pure substance requires, but this seems to 

 be accounted for by the formation of the above-described 

 basic compound. 



In aqueous oxalic acid the prussian blue is very easily so- 

 luble. By the addition of carbonate of potash to this liquid 

 the colour is changed to a red-brown, but no iron is precipi- 

 tated at the ordinary temperature ; as soon however as the 

 liquid is boiled, peroxide of iron falls down in the proportion 

 of three-fifths of that contained in the blue compound. 



An unweighed quantity of dried prussian blue was dis- 

 solved in oxalic acid, and by boiling with carbonate of potash 

 0*208 peroxide of iron were precipitated. The liquid con- 

 tained 0*137. The proportion of 3 to 2 requires 0*207 and 

 0*138. On a second determination 0*274 were precipitated 

 by potash, whilst 0*174 remained in solution. The propor- 

 tion of 3 to 2 requires 0*268 and 0*179. This reaction with 

 oxalic acid might reward further examination. A remarkable 

 circumstance connected with it is that the liquid after being 

 filtered off from the peroxide of iron, separated by potash, is 

 precipitated blue by muriatic acid. 



I have now endeavoured to describe the formation of prus- 

 sian blue under different circumstances, and the influence 

 which these exercise on its composition, giving particular at- 

 tention to the presence of potassium. 



It appears to me that this circumstance of the presence of 

 potassium is not a matter of mere scientific interest, but is 

 also of great importance to the dyer, for it materially affects 

 the colour and dyeing power of the product in the manner I 

 shall now proceed to state. The greater the quantity of potas- 

 sium contained in the cyanide, the lighter and more approach- 

 ing to violet is its colour ; and, on the other hand, in proportion 

 as the quantity of potassium is diminished the colour becomes 

 deeper and more powerful. Of the different blue compounds 

 described above the brightest and most striking is that 

 formed from the residue of the distillation of prussic acid, 



( 2Cfy-{ jr e j. Next to this in order stands the precipitate 



formed by pro to- sulphate of iron in an excess of ferridcyanide 



{5Fe 

 jr . The next link in the chain 



is a 



