1824.] 1'ests for detecting the Presence of Arsenic. 31 



confine myself to treating of the mode of employing the follow- 

 ing tests ; viz. the reduction of the arsenious acid to its metallic 

 state ; the use of sulphuretted hydrogen ; sulphate of copper ; 

 and nitrate of silver; and I hope to be able to describe 

 the little apparatus required in such a manner as to remove 

 some ambiguity and difficulty. 



Jt is justly observed by Dr. Paris (Jurisprudence, vol. ii. p. 

 252), that " the detection of the presence of arsenic amidst a 

 complicated mass of alimentary matter has long been a problem 

 of interest and difficulty," because " coloured fluids are capable 

 of obscuring and changing, and even altogether preventing the 

 arsenical indications." Dr. Paris then adverts to the proposal 

 of M. Orfilato destroy or modify the colouring matter by means 

 of chlorine; this method is justly stated to be liable to great 

 difficulties in its application, and from sources too obvious to 

 require notice. Dr. Paris, therefore, advises that a solution of 

 ammoniuret of silver should be added to the fluid to precipitate 

 indiscriminately all bodies which it may be capable of so affect- 

 ing ; and then subjecting the precipitate to the action of black 

 flux in a glass tube for the purpose of subliming the arsenic 

 in its metallic state. 



On considering this part of the subject, it appeared to me that 

 animal charcoal (ivory-black) might be advantageously employed 

 for the purpose of destroying the colouring matter. I there- 

 fore, mixed some of it with a coloured solution of arsenious 

 acid, viz. the liquor arsenicalis of the London Pharmacopoeia. 

 I found that the colouring matter was so completely destroyed 

 in a few minutes, that the test of nitrate of silver, or any other, 

 might be readily applied. I repeated this experiment with port 

 wine, gravy soup, and a strong infusion of onions *, and suc- 

 ceeded in these cases in procuring a solution sufficiently co- 

 lourless for the application of the most delicate tests. It might 

 be supposed that the phosphoric acid which the animal charcoal 

 contains might have some share in the production of the yellow 

 precipitate with silver ; I found, however, that water or wine 

 which was merely digested on the animal charcoal, produced 

 no effect with the nitrate of silver, excepting a slight precipitate 

 of chloride ; and to prevent this, the ivory-black should be 

 washed on a Alter with boiling distilled water, till the fluid run- 

 ning through ceases to be aflected by nitrate of silver. If, how- 

 ever, the ivory-black is of good quality, this precaution is un- 

 necessary. 



Of Sulphuretted Hydrogen. 



Supposing the substance suspected to contain arsenious acid, 

 to have been boiled in distilled water without any alkali, and 

 deprived of its colouring matter as now described, the test to 



• I particularly mention infusion of onions, because much stress has been laid uprft 

 the ambiguity which its yellow colour may occasion. It immediately becomes colourless 

 (Urn: Paris and i'onblanqne's Med. Jurisprudence, vol. iii. p. I'J'J.') 



