164 
Selected Articles. 
rounded with ice. Hence the absolute necessity of assaying 
in all such processes, the ultimate product, either by the ni- 
trate of silver or the peroxide of mercury method ; the first 
is to be preferred : we have the great advantage that any 
error committed in collecting, drying, and weighing, is re- 
duced to one-fifth in estimating the quantity of real acid, 100 
grains of the cyanide of silver corresponding to 20.38 of hy- 
drocyanic acid. 
(7.) In addition to the very elegant application of the ni- 
trate of silver for detecting the presence of free hydrocyanic 
acid in its passage as vapour from a dilute solution, or in any 
plant containing the acid, (thus, masticate a bitter almond, put 
it in watch-glass, and cover it with a bit of glass, on the un- 
der surface of which a drop of dilute nitrate of silver is 
placed ; in a few minutes the cyanide of silver is formed, — 
an experiment which may serve as a class illustration of the 
extreme volatility of the substance,) recommended by Mr. 
Barry in the London and Edinburg Philosophical Magazine, 
vol. iv. p. 151. Mr. Barry has also put me in possession of 
a means as elegant for the testing of the presence of minute 
quantities of hydrochloric or sulphuric acid in hydrocyanic 
acid, viz. Put some of the acid on a watch-glass, add two or 
three drops of liquor ammonise, put the glass on the sand- 
bath/and evaporate to perfect dryness, when all ammonia 
and hydrocyanic acid pass off, leaving only, if any hydro- 
chloric or sulphuric acid be present, a little hydrochlorate or 
sulphate of ammonia behind ; a drop or two of distilled wa- 
ter will dissolve these, and by nitrate of silver added to one- 
half, and nitrate of barytes to the other, the presence or ab- 
sence of the above acids will be determined. If the hydro- 
cyanic acid be quite pure, the watch-glass after evaporation 
is scarcely soiled, and water dissolves nothing: this method 
is far preferable to that by means of carbonate of lime usually 
recommended. 
(8.) In a paper which I read to the Medico-Botanical So- 
ciety, on Tuesday, Dec. 9, 1834, on the methods of assaying 
medicinal hydrocyanic acid, I stated that I had examined 
samples of the acid procured from various shops in town, and! 
