Ferrocyanuret of Potassium and Sulphuric Acid. 101 



kept very cold. But no process for procuring a dilute solu- 

 tion 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 7iecessity of assaying in all such processes the ultimate 

 product, either by the nitrate of silver or the peroxide of mer- 

 cury method ; the first is to be preferred : we have the great 

 advantage that any error committed in collecting, drying, and 

 weighing, is reduced to ^ in estimating the quantity of real 

 acid, 100 grains of the cyanide of silver corresponding to 

 20-38 of hydrocyanic acid. 



(7.) 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 in any plant 

 containing the acid, (thus, masticate a bitter almond, put it in 

 a watch-glass, and cover it with a bit of glass, on the under sur- 

 face 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. 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 am- 

 moniae, put the glass on the sand-bath, and evaporate to per- 

 fect dryness, when all ammonia and hydrocyanic acid pass off, 

 leaving only, if any hydrochloric or sulphuric acid be pre- 

 sent, a little hydrochlorate or sulphate of ammonia behind; a 

 drop or two of distilled water will dissolve these, and by ni- 

 trate of silver added to one half, and nitrate of barytes to the 

 other, the presence or absence of the above acids will be de- 

 termined. If the hydrocyanic 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 car- 

 bonate 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 sam- 

 ples of the acid procured from various shops in town, and that 

 the frightful difference of strength had induced me to make 

 the results known, with a view of calling the attention of the 

 medical profession to the evil. Thus, samples from Allen, 

 Hanbury and Co. yielded 5*8 per cent.; from Apothecaries' 

 Hall, at different times, from 2* 1 to 2*6 per cent. ; and from 



