486 
DR. PLAYFAIR ON THE NITROPRUSSIDES, 
tense, and enables the detection of the most minute quantity of either reagent. As 
a test for the presence of sulphides it is wonderfully useful, enabling minute quan- 
tities of them to be found in circumstances where the ordinary means of testing alto- 
gether fails to denote their presence. This purple coloration is however only transi- 
tory, the compound soon breaking up into various substances, among which, hydro- 
cyanic acid, ammonia, nitrogen, oxide of iron, a ferrocyanide, a sulphocyanide and a 
hyponitrite may be recognized. 
The soluble nitroprussides are decomposed when sulphuretted hydrogen is passed 
through them, oxide of iron, prnssian blue, sulphur, a ferrocyanide, and a peculiar 
sulphur compound being among the products of decomposition. 
The alkalies decompose the soluble nitroprussides when their solutions are mixed 
together and boiled. The products of the transformation in this case are oxide of 
iron, nitrogen, a ferrocyanide and a hyponitrite. An excess of ammonia, even in the 
cold, gradually decomposes the nitroprussides, nitrogen gas being evolved, and a 
peculiar uncrystallizable black compound remains as the result of the decomposi- 
tion. 
Sulphurous acid, the sulphites and hyposulphites exert no apparent action on the 
nitroprussides. They are however wholly decomposed by boiling them with con- 
centrated sulphuric acid ; during this decomposition, the peculiar purple colour due 
to sulphides is observed. 
Chlorine does not produce any change when passed through solutions of the nitro- 
prussides. 
Prussian blue dissolves in an excess of some of the nitroprussides, forming a beau- 
tiful blue solution ; when the prussian blue is in excess, it is able, under certain 
circumstances (see § 5), to remove the soluble nitroprusside from solution, but it 
again yields it up to boiling though not to cold water. 
Some of the nitroprussides are very permanent and suffer no change in solution, 
either by exposure to the air or by the action of heat. Several, on the contrary, espe- 
cially nitroprussic acid, the nitroprussides of barium, calcium and ammonium, de- 
compose 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 unde- 
composed 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 hydrochloric acid, or by precipitating nitroprusside of 
barium with an equivalent quantity of sulphuric acid. It may also be obtained, 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 alco- 
