Mr. Smee on the Ferrosesquicyanuret of Potassium. 199 



from the mass, a result which is known by its not discolouring 

 starch upon the addition of nitric acid, it still retains its 

 power of forming Prussian blue with salts of either oxide 

 of iron, and still presents the same indisposition to crystallize, 

 for it neither shows itself as the yellow nor the red ferrocy- 

 anate of potash, but as a compound having properties inter- 

 mediate with both. 



When iodide of potassium is added to the ferrosesquicy- 

 anuret, iodine is evolved, the solution loses its red colour, and 

 the salt possesses the characters similar to the mass obtained 

 by the action of iodine on the ferrocyanate of potash. Thus 

 it is evident that if a solution of persulphate of iron be treated 

 with the red ferrocyanate whilst an iodide is present, Prussian 

 blue will be formed. 



Whether this is really a mixture of the ferrocyanuret and 

 ferrosesquicyanuret or a distinct compound, it is difficult to 

 determine, but the latter is rendered probable from its ge- 

 nerally presenting itself as an amorphous mass ; yet, however, 

 when the purified mixture is dissolved two or three times in 

 water, a dark mass is deposited, and at last crystals of the yel- 

 low salt are formed. 



Every method which has been discovered of converting the 

 ferrocyanate of potassa into the ferrosesquicyanuret has now 

 been detailed, and we have seen that they may each be re- 

 ferred to the class of anions, for of the cathions the powerful 

 agency of potassium was unable to effect this change. 



Upon the first formation of the ferrosesquicyanuret the 

 colour will occasionally be a very dark red, but this is an ad- 

 ventitious, not a necessary property ; for when prepared by 

 peroxide of manganese or chloride of soda, it does not possess 

 this dark colour. If the red crystals be carefully picked and 

 re-dissolved, in no instance is this seen, and in every case where 

 the dark red exists it yields to liquor ammoniae or potassae, 

 with the production of a small quantity of the ferrocyanate. 



The ferrosesquicyanuret, however prepared, has the same 

 peculiar properties. It has been already mentioned that the 

 protosalts are precipitated blue, whilst the persalts are not 

 effected by this agent ; however, the solution in the latter case 

 is" always much darkened, and after a time a small quantity 

 of dark- coloured substance is deposited. The mode of pre- 

 paration of the ferrosesquicyanuret does not influence this re- 

 sult. 



With almost every acid, especially if heat be applied, Prussian 

 blue is formed and hydrocyanic acid is given offj and thus 

 upon testing for minute quantities of metal, care must be taken 

 to prevent any excess of acid, as in that case the chemist would 



