1819.] Cyanogen and Hydrocyanic Acid. 433 
liquor, which is speedily followed by ammonia ; and the brown 
residue, when dissolved in muriatic acid, forms a yellow solution, 
in which potash produces a precipitate of the same colour. 
M. Vauquelin does not say positively whether this prussiate be 
a hydrocyanate ; yet as the affinity of copper for oxygen 1s not 
very strong, it is very natural to think that when the oxide of 
copper unites with hydrocyanic acid, it produces a cyadide. But 
the colour of this substance is not that of the red prussiate 
obtained by the action of triple prussiate of potash, or sulphate 
of copper. But as this prussiate contains prussian blue, it is 
possible that this last substance has some influence on the 
colour. 
On the Prussiate of Copper. 
The prussiate of copper is of a fine red colour. It is very 
bulky while moist. hen treated with ammonia its volume 
- diminishes very much. it loses its colour, becomes greenish- 
yellow, and assumes a crystalline form. The ammonia, in which 
the prussiate is digested, is scarcely tinged green; though it 
contains a little copper. When diluted with water and kept in 
a well-corked phial, it allows, after some time, a beautiful orange- 
coloured matter to fall down. 
The prussiate of copper, rendered green by ammonia, when 
put in contact with water, immediately recovers its original 
clon and this phenomenon may be renewed as often as we 
please. 
M. Vauquelin concludes from these interesting facts, 1. That 
the common prussiate of copper is a hydrate ; 2. That ammonia 
merely deprives it of the water which it contains; 3. That its 
red colour is owing to water, and that its natural colour is 
green. 
M. Vauquelin observes that it is remarkable to see this alkali 
having no other action on common prussiate of copper but that 
of abstracting its water; while, on the other hand, it is a good 
solyent of the simple prussiate of copper. 
Action of Cyanogen on the Oxide of Iron and on Metallic Iron. 
In the researches that follow, M. Vauquelin examines a diffi- 
cult question, not hitherto answered ; namely, whether prussian 
blue be a hydrocyanate or a cyanide. This celebrated chemist 
having formed an opinion on the subject founded on experiment, 
we shall not be hereafter under the necessity of forming vague 
ideas respecting a substance so generally known and so useful. 
From the preceding part of this article, it will be easy to see 
what happens when cyanogen dissolved in water is placed in 
contact with oxide of iron or with metallic iron. M. Vauquelin 
enumerates the phenomena which he observed, and he concludes 
from it that cyanogen, when in contact with oxide ofiron, under- 
goes the same changes as in water alone, but with greater rapi- 
ity: that ammonia, carbonic acid, and hydsocyanic acid, are 
Vou. XIII. N° VI. 2E 
