332 Essaij upon the Compounds of Cyanogen. 



( cyanogen, 52 ^ 

 FeiTO-cyanic acid, < iron, 28 ^81 = 1 atom. 



( hydrogen, 1 ) 



Potassa, - - 5 potassium, 40 j^g_ J ,, 

 I oxygen, 8 3 



Water, - - \^xy^^^, 8> ^^^ , 

 ( hydrogen, 1 ) 



1 38 representative number. 



This salt when exposed to heat gives off, as above stated, 13 per 

 cent, of water, a quantity as nearly as possible equivalent to two pro- 

 portionals, one of which may be regarded as water of crystallization, 

 and the other as formed by the union of the hydrogen of the acid 

 with the oxygen of the base. The residue contains therefore two 

 atoms of cyanogen, one of iron and one of potassium. Supposing 

 the cyanogen to be equally divided between the metals, it will con- 

 sist, in the dry state, of 



Cyanuret of iron, _ - - 264-28 = 54=1 atom. 

 Cyanuret of potassium, - - 26 + 40 = 66 = 1 " 

 As the ferro-cyanate of potassa has lately become an article of com- 

 merce, and is extensively employed in some of the arts, it is manu- 

 factured on a large scale, by igniting a mixture of animal matter and 

 carbonate of potassa in iron crucibles, until the dense and fetid va- 

 pors at first evolved cease to be extricated. The dry mass when 

 cold is dissolved in v/ater, and by repeated solution, filtration and 

 crystallization, the salt is obtained in a state of sufficient purity for the 

 purposes to which it is to be applied. 



Per-ferro-cyanate of iron — Prussian blue. — This well known sub- 

 stance was accidentally discovered by Diesbach and Dippel, at Ber- 

 lin, in the year 1704. The process for obtaining it remained, how- 

 ever, a secret until, in 1724, Woodward published an essay in the 

 Philosophical Transactions, containing an account of its preparation 

 and of many of its properties. The attention of chemists, which 

 was thus directed to its investigation, was entirely unsuccessful, un- 

 til, in 1752, Macquer pronounced it to be a compound of oxide of 

 iron, with a peculiar coloring principle, which being unable to insu- 

 late, he considered as phlogiston. So enamored were the chemists 

 of that day with the absurdities of the phlogistic hypothesis, that this 

 opinion was immediately and universally adopted. After a long in- 

 terval, Guyton and Bergmann suggested, and Scheele succeeded in 

 proving, that this compound is a salt. Since that period it has sue- 



