176 APPENDIX. [No. VII. 



the solution becomes dark-coloured. This liquor, when neutralized with 

 potassa, is found to give no precipitate with the persalts of iron, but forms 

 Prussian blue with the protosalts of that metal. The rapidity of this 

 change depends upon the heat of the solution, for when warm the effect 

 takes place immediately, whilst, on the contrary, two or three days are 

 required at a low temperature. When evaporated, a large quantity of 

 nitrate of potassa is deposited ; and, lastly, some red crystals are formed. 

 When acid is more used, the f errocyanate is totally decomposed ; the black 

 mass which is the result has at first a sweet, but afterwards leaves a dis- 

 agreeable metallic taste upon the palate. This process can never be used 

 advantageously to form the ferrosesquicyanuret, from the quantity of acid 

 which is required, the degree of nicety which must be employed to effect 

 the change, and the impurity of the salt when obtained. 



The next highly-oxygenated acid which we have to examine is the 

 iodic : this when added to ferrocyanate of potash becomes decomposed, 

 the oxygen passes to the potassium to form potassa, free iodine is evolved, 

 and the potassa passes to another portion of iodic acid, and is precipitated 

 as the iodate of potassa. The free iodine can be readily removed by agita- 

 tion with a little ether, and in this way a tolerably pure ferrosesquicy- 

 anuret of potassium can be extemporaneously obtained, for the solution 

 contains but little iodate of potassa from its insolubility.* 



Chloric acid operates in the same way as iodic acid, but is more diffi- 

 cult of decomposition, and it requires the action of heat before the smell 

 of chlorine is exhaled and the red ferrocyanate formed. 



If chlorate of potassa be added to the ferrocyanate, and dilute sul- 

 phuric acid be dropped into the solution, red ferrocyanate of potash will 

 also be formed. 



Bromic acid will not act upon the ferrocyanate with the production of 

 the ferrosesquicyanuret, but acts as other acids in forming Prussian blue. 

 A great variety of other oxyacids have been tided, but none were 

 found to part with their oxygen. 



When a large quantity of peroxide of manganese in fine powder is 

 added to a solution of the ferrocyanate of potash, and the mixture digested 

 for a considerable time, the ferrocyanate becomes converted into the 

 ferrosesquicyanuret, and on evaporation crystals of the most beautiful 

 ruby red are obtained. The salt thus procured appears to be very pure. 



If a little dilute sulphuric acid be added to the solution in conjunction 

 with the peroxide of manganese, the action takes place more quickly, but 

 sulphate of potassa is formed, which is a great disadvantage. 



The last process in which nascent oxygen contributes to the formation 

 of ferrosesquicyanuret of potassium, is, perhaps, one of the most elegant, 

 efficient, and simple processes in the whole range of chemistry. This 

 mode I was induced to follow from the consideration, that as nascent 

 oxygen effects a change of the yellow to the red ferrocyanate of potassa, a 

 similar change must be produced by its being subjected to a galvanic 

 current. Accordingly some solution of the salt was placed in a tube bent 



* This elegant process can be employed with advantage when a small 

 quantity of the salt is suddenly wanted, as it scarcely requires a minute to 

 effect. 



