No. VII.] APPENDIX. 175 



smell of chlorine, and is changed from a yellow colour to a dark red, and 

 deposits on evaporation red crystals. A similar change takes place when 

 bromine is added to the ferrocyanate, and in both cases the weight of the 

 entire red mass is equal to that of the yellow ferrocyanate, plus the weight 

 of the chlorine or bromine used, but minus the quantity of water which 

 the yellow crystals are known to contain. This indicates, first, that the 

 red crystals are anhydrous ; and, secondly, that the chlorine or bromine is 

 actually absorbed by the salt. The former fact is confirmed by heating 

 the red precipitate in a test tube, when no water is given off ; and the 

 latter fact is also proved by the evolution of chlorine or bromine, on the 

 addition of two or three drops of strong heated sulphuric acid to a few 

 grains of red salt. 



When heated alcohol is added to this red mass, a small portion is dis- 

 solved, which is again deposited when the spirit is evaporated. This salt 

 by its characters is known to be either the bromide or the chloride of 

 potassium. By this method the red ferrocyanate of potassa, which is 

 insoluble in alcohol, becomes purified: but this is a troublesome and 

 expensive process, as the bromide or chloride is but little soluble in the 

 spirit, and therefore a large quantity must be used. 



About half an equivalent of chlorine or bromine is required to effect 

 this change, and great care must be employed to prevent excess of these 

 substances, as they are apt to react upon a portion of the salt. The liquid 

 in this case contains Prussian blue dissolved, which materially discolours 

 the salts, and it can only be precipitated from the solution by the addition 

 of neutral salts, as sulphate of soda, which renders the red ferrocyanate 

 impure. In a similar manner, chloride of soda, as might be expected, 

 forms the red ferrocyanate of potassa. 



Prom the foregoing details a knowledge is obtained of the action of 

 chlorine and bromine upon the ferrocyanate, for we have seen that chloride 

 and bromide of potassium is formed, and that one-half an equivalent of 

 these substances is necessary for this change. Now it is manifest that 

 half an equivalent of potassium is removed from the ferrocyanate, so that 

 the new salt, instead of consisting of iron one equivalent, potassium two 

 equivalents, cyanogen three equivalents, contains iron one equivalent, 

 potassium one and a-half equivalent, cyanogen three equivalents; and 

 therefore it is rightly named the ferrosesquicyanuret of potassium: that 

 half an equivalent of potassium has been removed from the salt, two or 

 three experiments have verified. 



The acids as a class will not effect a similar change, because as they 

 combine not with potassium but with potassa, water must be decomposed, 

 the oxygen uniting with the metal, and the hydrogen passing to the 

 ferrocyanate, forming hydroferrocyanic acid. 



A question naturally arises whether the potassium may not be removed 

 from the ferrocyanuret by other processes, and we are led to try the 

 action of the anions, and of these I attempted to add oxygen to the salts 

 by the use of nitric acid. This acid, when added in small quantities to 

 the yellow ferrocyanate, acts as the other acids by liberating hydrofen-o- 

 cyanic acid, which is speedily decomposed into a pale bluish cyanuret of 

 iron. When, however, further additions of this acid are made, the potas- 

 sium takes oxygen, forms potassa, deutoxide of nitrogen is evolved, and 



