HYDROCYANIC ACID. 
237 
This cyanuret is not easily affected by light, for I have left 
it in the experiment glass and under water exposed during 
three months to ordinary light, and although it had assumed 
a grayish violet tint, it retained all its properties. 
The hydrocyanic acid being once fixed in this way by an 
insoluble combination, the volume of which permits the de- 
tection of the smallest quantities ; there are, then, many me- 
thods of proving really the presence of this acid, either by 
treating it with muriatic acid, which disengages the odour of 
hitter almonds^ or by transforming it by means of corrosive 
sublimate into a cyanuret of mercury, crystallizing in beau- 
tiful prisms and soluble in water, &c. These proofs are 
unquestionably easy to obtain when sufficiently notable 
proportions are procured, but the same is not the case when 
these proportions are very minute. I believe that under such 
circumstances the prussic smell is too feeble, and of too short 
duration, to be perceived certainly by our senses; besides, 
the formation of the cyanuret of mercury presents other dif- 
ficulties. To obviate this inconvenience, I have thought 
that reactions capable of producing prussian blue or red 
ferrocyanate of copper, offered a great advantage. M. 
Lassaigne had already presented a method which detected 
traces of hydrocyanic acid in a liquid, but as this method did 
not always furnish decided reactions, I have endeavoured to 
substitute for it another process, — which appears to me to 
conduct to a result more determined. It consists in trans- 
forming in a few moments, and without difficulty, the cyanuret 
of silver into a soluble ferrocyanate, which can give blue or 
reddish brown precipitates with the salts of iron at a maxi- 
mum, and with those of copper. 
It was necessary, before putting this method or modification 
into operation, to be certain of the sensibility of the reagent 
employed, and I have done this in the following manner: — 
gr, 1 decigramme of ferrocyanate of potassa was dissolved 
the other liquid. But when the quantity of water is considerable, the 
degree of volatility of the organic compound is so diminished as to be 
easily modified by the water and heat. 
VOL. III. NO. III. 30 
