236 
SELECTED ARTICLES. 
ART. XL.—CHEMICO-LEGAL RESEARCHES, TO DETER- 
MINE THE PRESENCE OF VERY SMALL QUANTITIES 
OF HYDROCYANIC ACID— EITHER FREE OR COMBINED. 
By M. OssiAN Henry. 
When researches are instituted having legal chemistry as 
their object, the slightest modifications ought to be published 
which can render processes in use more precise and certain 
in their execution; with this motive I have determined to 
present those which 1 have made, in order to appreciate with 
exactness the very minute quantities of hydrocyanic acid, 
free or combined, contained in different products. Having 
had occasion to inquire into the existence of hydrocyanic 
acid, I have been induced to believe that of all the methods 
published, either recently or long since, that most advanta- 
geous beyond doubt, for the purpose of fixing and seizing this 
principle, consists either in precipitating it directly by the 
nitrate of silver, or in volatilizing it by distillation ; or after 
having eliminated it from its compounds, receiving the vola- 
tile product in a weak solution of this salt very pure. The 
hydrocyanic decomposes this last and gives origin to a white 
curdled deposit of considerable amount insoluble in water_, 
and in weak nitric acid, and which consists of cyanuret of 
silver. 
* A liquid containing hydrocyanic acid (prepared recently by the me- 
thod of Gea Pessina) was separated into two equal parts A. and B. The 
first, A., furnished by distillation dry cyanuret of silver 3.10. The second 
B., by direct precipitation dry cyanuret of silver 3.00 in weight, almost 
the same. 
This experiment is at variance with that obtained under circumstances 
almost similar, where the quantity obtained by distillation was found in 
the least proportion, probably on account of the transformation of the 
prussic acid into formic. I think that the proportion of water to the 
prussic acid was, in this case, so small, that the latter was enabled to 
disengage itself promptly without undergoing any action on the part of 
