a New Class of Salts, 209 



action of heat. Several, on the contrary, especially nitro- 

 prussic acid, the nitroprussides of barium, calcium and am- 

 monium, decompose partially, either when their solutions are 

 long kept, or speedily when they are boiled. Some of the 

 products of decomposition are dissolved by the still undecom- 

 posed nitroprusside, and cannot be again separated from them 

 by crystallization. 



After this general idea of the habits of the nitroprussides, 

 their individual salts and their transformations may be more 

 easily studied. 



Nitroprussic Acid. 



10. This acid may be obtained in solution by decomposing 

 nitroprusside of silver with an equivalent quantity of hydro- 

 chloric acid, or by precipitating nitroprusside of barium with 

 an equivalent quantity of sulphuric acid. It may also be ob- 

 tained, but in a less pure state, by precipitating nitroprusside 

 of potassium dissolved in a small quantity of water, and diluted 

 with several times its volume of alcohol, with an alcoholic 

 solution of tartaric acid, the quantity of the latter being just 

 sufficient to form bitartrate of potash with the potassium ; but 

 as the acid dissolves some of the latter salt, this process does 

 not yield a pure product. 



A dark red-coloured solution, strongly acid, is obtained by 

 these methods, .^ther does not precipitate the acid as it does 

 ferrocyanic acid. Soon however the solution begins to form 

 hydrocyanic acid, and either to deposit oxide of iron or to 

 hold iron in solution, which may be detected by a prusside. 

 When this change has taken place, evaporation in vacuo over 

 sulphuric acid yields crystals of the acid, which is however 

 found to contain a small quantity of an impurity, probably of 

 a cyanide of iron, which cannot be separated by crystallization, 

 or any other of the numerous methods tried. The amount of 

 this impurity is from 2 to 3 per cent. This crystalline acid 

 belongs to the oblique system, and its crystals are described 

 and measured in a further part of this paper, together with 

 its analyses. It possesses all the properties of nitroprussic 

 acid, and only differs by containing this small quantity of im- 

 purity. The perfectly pure acid in crystals has not been ob- 

 tained, notwithstanding very many efforts to obtain this desi- 

 rable result. 



Nitroprusside of Sodium. 



11. This salt is the most readily procured, in a crystallized 

 state, of all the nitroprussides ; it may be obtained by decom- 

 posing the nitroprussides of copper or iron by means of soda, 



Phil, Mag, S. 3. Vol. 36. No. 242. March 1850. P 



