Bine Compounds of Cyanogen and Iron. 169 



compound formed by the decomposition of ferridcyanide of 

 potassium by sulphuric acid, which contains still less potas- 

 sium, and which I will presently describe. Darkest of all 

 are the compounds which contain no potassium. The dyeing 

 power of these substances, by which term I intend to desig- 

 nate the relative power of imparting a blue colour to any co- 

 lourless substance with which they may be mixed, is in the 

 inverse ratio to their quantity of potassium ; but in such a 

 manner that those containing potassium have a smaller co- 

 louring power than, after deducting the cyanide of potassium 

 contained in them, would belong to the remainder ; whence 

 we see that the potassium plays no indifferent part in their 

 colouring power. 



When a solution of ferridcyanide of potassium is treated 

 with concentrated sulphuric acid a green precipitate is formed, 

 consisting of percyanide of iron and a small quantity of cyanide 

 of potassium. By continued boiling with an excess of sul- 

 phuric acid its colour is changed into a rich. blue. It is ad- 

 visable to continue boiling for a considerable time after the 

 change of the colour has taken place, in order to ensure the 

 complete transformation. Ammonia, which may be easily 

 proved to exist in the liquid, is here formed at the expense of 

 a part of the cyanogen. This compound was washed and 

 dried in the manner described in former instances, and gave 

 on analysis to 1*962 peroxide of iron 0*049 sulphate of potash, 

 which is one proportion of potassium to about sixty iron, too 

 small a quantity of potassium to be considered as essential. 



I. 1*464 grm. gave 0*702 peroxide of iron, corresponding 

 to 33*25 per cent. iron. 



II. 1*152 grm. gave 0*556 peroxide of iron, corresponding 

 to 33*45 per cent. iron. 



III. 0*549 grm. gave 0*266 peroxide of iron, corresponding 

 to 33*61 per cent. iron. 



Burned with chromate of lead, — 



I. 0*481 grm. gave 0*306 carbonic acid and 0*131 water, 

 corresponding to l/'3 per cent, carbon and 27*2 water. 



II. 0*347 grm. gave 0*219 carbonic acid and 0*95 water, 

 corresponding to 17*7 per cent, carbon and 27*0 water. 



The following table shows the degree of approximation to 

 the formula Fe 5 Cy 6 . + 13Aq : — 



Phil. Mag. S. 3. Vol. 29. No. 1 93. Sept. 1846. N 



