286 Means of detecting Arsenic in the Animal Body, ^c. 



peroxide being left. If we still retain the iron in the apparatus, 

 but make the action not very brisk, no tache will exhibit itself 

 upon a smooth porcelain surface ; but if the broken surface of a 

 piece of porcelain is placed in contact with the flame, a slight 

 black deposit is formed, consisting, as in the former case, of per- 

 oxide of iron ; the reason of this is, that it is a more convenient 

 surface for retaining the particles of the solution of iron thrown 

 out in company with the hydrogen. Again, this tache is evi- 

 dently an oxide, which it is not probable would be the case, had 

 the iron been chemically combined with the hydrogen. Another 

 reason is, that if the gas be made to traverse water orchloride of 

 calcium before ignition, no tache will be formed, for the iron 

 mixed with the hydrogen is retained by either of these means. 

 This tache has been perhaps more noticed than it deserves. It is 

 not easily produced, and is distinguished by its not being volatile 

 and its solution in any of the strong acids, giving a blue precipi- 

 tate with ferrocyanuret of potassium. 



There is yet one other tache to be spoken of. If the flame of 

 the apparatus, containing only zinc, sulphuric acid and water, be 

 prolonged for some time upon one spot on the porcelain, an opake 

 white tache will be perceived, which I propose to explain in the 

 same way as the last, the cause of it being the oxide of zinc 

 instead of iron, this oxide arises from the decomposition of a 

 small quantity of sulphate of zinc thrown out with the hydrogen, 

 but still it is a thing hardly worthy of notice, for after it is formed 

 it is difficult to see it. 



What is the conclusion to be arrived at concerning Marsh's ap- 

 paratus, after what has been said ? Why, that it should be con- 

 sidered as the most valuable instrument that the medico-jurist 

 possesses, to assist him in his experiments upon the poison in 

 question ; for with proper care all the objections to it can be easily 

 remedied, and the character of each tache is so well marked that 

 they need never be confounded, as will be seen by glancing the 

 eye over what follows. 



Arsenic — Steel color, highly metallic, easily volatilized by heat, 

 readily dissolved in nitric acid ; the nitric acid solution gives with 

 nitrate of silver a brick-red precipitate. 



Antimony — Color darker than steel, metallic, with difficulty 

 volatilized by heat, readily dissolved in nitric acid ; the nitric acid 

 solution gives with nitrate of silver no precipitate. 



