of uniform Strength from Cyanide of Silver, 103 



certained, and every five grains of it will yield one grain of acid. 

 It can be procured by conducting the vapour from the process 

 described in section (6.) of this paper, into a pint of water, hold- 

 ing 255 grains of nitrate of silver, washing and drying at 212°. 

 It yields 201*6 grains of white cyanide. 1 should recommend 

 that the bottle containing this salt be accompanied by a small 

 stoppered phial with dilute hydrochloric acid of such strength, 

 that 1 minim will exactly decompose 1 grain of the cyanide: 

 thus, suppose one corked phial having 200 grains of cyanide 

 with one |-oz. stoppered bottle with hydrochloric acid of 

 specific gravity 1*129, this would be enough to make 5 fluid 

 ounces of dilute hydrocyanic acid of the Dublin strength, if 

 the following formula be followed. Into a phial capable of 

 holding rather more than 1 fluid ounce, put 40 grains of the 

 cyanide, add 7 fluid ounces 20 minims of water, and 4-0 mi- 

 nims of the dilute hydrochloric acid; cork closely, shake se- 

 veral times for the first quarter of an hour, set aside to allow 

 the chloride of silver to fall, decant the clear liquid into ano- 

 ther bottle to be preserved for use : every fluid drachm will 

 contain 1 grain of real hydrocyanic acid. 



The only objection I had a priori to this process was the lia- 

 bility of a little free hydrochloric acid remaining in the solu- 

 tion, since all books echo that the presence of a minute quan- 

 tity of the mineral acids very much hastens the decomposition 

 of this acid ; a statement perfectly opposite to fact, at least as 

 far as concerns hydrochloric acid. I prepared 4 ounces of 

 hydrocyanic acid perfectly pure by distillation off* chalk; to 2 

 ounces I added 5 drops of hydrochloric acid ; the other two 

 ounces in another phial were left perfectly pure, both inverted 

 and placed in a glass case so as to have diffused light during 

 the day. After three weeks the pure acid had become quite 

 brown, and a considerable quantity of solid deposit had formed ; 

 the other remained quite limpid and colourless, and on actual 

 trial was found to contain |^§ths of the acid which it had at first. 

 Mr. Barry also informed me that his fourteen years' expe- 

 rience led to the same result ; and that, being aware of this, 

 he adds purposely a little hydrochloric acid to all his medi- 

 cinal acid. Perhaps some may object to the price of the pre- 

 paration : a case containing the two bottles with 200 grains 

 of the cyanide would leave one half profit if sold for 5s, ; this 

 brings an ounce of acid to 1 5., and where so small a quantity 

 is used, surely this cannot be a very weighty objection, if a 

 uniform article can be secured. 



28, Golden-square, London, Dec. 12, 1834. 



