240 
SELECTED ARTICLES. 
solution of nitrate of silver, afforded in all three instances a 
light gray powder, in which the existence of hydrocyanic 
acid was more decidedly proved by the method I shall now 
indicate. 
Frocess for appreciating very minute quantities of hydro- 
cyanic acid, free or combined. 
When a liquid, or any product whatever, is supposed to 
contain hydrocyanic acid, either in a free state, or in combina- 
tion with a base, it is necessary either to precipitate it at once 
by a weak solution of nitrate of silver,or if the liquid is coloured 
and mixed with foreign salts, to distil it in a glass vessel well 
adapted for the purpose, adding water only in the first in- 
stance, or acidulated with muriatic in the second, the compound 
being a little concentrated before handj the volatile product 
is received into a solution of nitrate of silver very much 
diluted, and soon forms a turbid white precipitate .which 
subsides to the bottom of the vessel; when the disengagement 
no longer produces any cloudiness in the solution, the opera- 
tion is to be arrested and the precipitate separated. This is 
washed in distilled water, when it is collected carefully in 
order to be moderately heated with one-half its weight of 
common salt ;* it is then allowed to cool and filtered; some- 
times the liquid is a little cloudy from the presence of a small 
quantity of alkaline cyanuret, which dissolves the chloride of 
silver. To the filtered product is added a small quantity of 
hydrated oxide of iron, (made by precipitating the proto- 
sulphate of iron in solution by potassa,) which is greenish, 
being a mixture of protoxide and peroxide; it is filtered again 
after being moderately heated, and the least traces of ferro- 
cyanate of soda being present (a salt which is produced in 
the preceding reactions) arising from the original prussic acid; 
a blue colour is obtained by the addition of a few drops of a 
solution of the muriate or sulphate of the peroxide of iron. 
After a few hours this colouring disappears and gives place to 
* For this salt may be substituted, either muriate of potassa, or muriate 
of lime, or magnesia. 
