A NEW CLASS OF SALTS. 
499 
acid and the nitroprussides of ammonium and calcium, have not yet been obtained 
free from this impurity, and are therefore described in this section. 
Nitroprussic Acid. 
19. The mode of preparation of this acid has been already described at page 486. 
It is however most readily prepared from nitroprusside of silver by adding to it as 
much hydrochloric acid as suffices to form chloride of silver with the silver in the 
salt. The dark red solution thus obtained soon evolves hydrocyanic acid, even in 
the cold, and after a time prusside of potassium indicates the presence of iron in solu- 
tion. If the solution be heated, it deposits abundance of a brown precipitate resem- 
bling oxide of iron. When the latter is separated by filtration, and the solution is 
evaporated in vacuo over sulphuric acid, crystals are formed and may be separated ; 
they must be dried over sulphuric acid, as they are exceedingly deliquescent. These 
crystals belong to the oblique system, but on account of their excessive tendency to 
deliquesce, it is difficult to measure their angles with accordant results. The angles 
between normals to the only faces which gave results to be depended on, are stated 
by Prof. Miller to be as follows : — 
ec 36 57 
e'c' 36 57 
ee' 1 06 6 
It will be seen that the equality of the angles ec and e’c' is a tolerably certain indi- 
cation that the crystals belong to the oblique system. 
The acid made by the action of hydrochloric acid on nitroprusside of silver, and 
evaporated over sulphuric acid in the cold, crystallized (light being excluded) without 
the deposition of oxide of iron, but the smell of hydrocyanic acid, accompanied by a 
peculiar pungent smell, was strongly perceptible. Analysis shows that these crystals 
are the same as those obtained from a boiled solution. 
Properties of the Crystallized Acid . — The crystallized acid is of a dark red colour, 
and has a very acid reaction, the crystals being generally flattened and of tolerable 
size. They are quite as deliquescent as chloride of calcium. They dissolve to a large 
extent in water, and are also soluble in alcohol and in ether. They may be dried in 
the water-bath without change, but their aqueous solution cannot be boiled without 
decomposition. 
The following analyses were made on crystals obtained from a boiled solution, and 
were dried at 212°. The acid was that made by the action of hydrochloric acid on 
the silver salt. Nos. I. II. and III. were preparations made at distinct times. 
The iron w^as determined by calcination and by treating the residual oxide with 
nitrate of ammonia. 
I. 2'345 grs. gave 0'800 gr. peroxide of iron. 
II. 3’915 grs. gave T325 gr. peroxide of iron. 
III. 3'580 grs. gave T220 gr. peroxide of iron. 
