HYDROCYANIC ACID, &C. 
161 
ART. XXIV.— PREPARATION OF HYDROCYANIC ACID OF 
UNIFORM STRENGTH. By Thomas Everitt, Esq., Professor of 
Chemistry to the Medico-Botanical Society. 
The best proportions of the ferrocyanuret of potassium and 
sulphuric acid to be used when we want hydrocyanic acid, are 
as follow: — 
To every .212.47 grains of the crystals dissolved in about 
two fluid ounces of water, add so much dilute sulphuric acid 
as shall contain 120 grains of real acid, and, by conducting the 
distillation carefully, 41 grains of hydrocyanic acid pass off, 
and that I find with the first third of the water; of course 
water must be put into the receiver and kept very cold. But 
no process for procuring a dilute solution of hydrocyanic acid, 
in which distillation or filtration is had recourse to, will yield 
an acid of uniform strength, however carefully the process 
may be conducted, not even, as I have proved, if the receiver 
be surrounded with ice. Hence the absolute necessity of 
assaying in all such processes the ultimate product, either by 
the nitrate 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. 
In addition to the very elegant application of the nitrate of 
silver for detecting the presence of free hydrocyanic acid in 
its passage as vapour from a dilute solution, or any plant con- 
taining the acid (thus, masticate a bitter almond, put in a watch 
glass, and cover it with a bit of glass, on the under 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 Edinburgh Philosophical Magazine, vol. iv., p. 151, (or 
VOL. II.' — NO. II. 21 
