NATURE AIJD CETECTION OF METALLIC POISONS. 307 



.'stake, to swear positively, that arsenic has been detected in his 

 operations. The alitali employed, whether arsenic be present 

 or not, unjforinly occasions a precipitate, by detaching the 

 oxide of copper from its combination with the sulphuric acid. 

 The colour of the deposit, it is true, is not absolutely the same 

 in both cases; but, when it is recollected, that experiments of 

 this kind are, for the most part, conducted on solutions coloured 

 to a greater or less degree from the matters found in the sto- 

 mach or intestines, this minuteness of distinction will be deemed 

 scarcely appreciable by the eye even of the most experienced 

 operator. Such uncertainty ought, most assuredly, not to at- 

 tend investigations of this nature; and the process of Dr. Bos- 

 tock, therefore, becomes objectionable from this circumstance. 



The method of detecting arsenic next deserving of remark. Process lately 

 js one lately recommended by Mr. Hume, of London. It recommended 

 consists in adding a quantity of subcarbonate of soda to a sola- '' " '^' ""^^* 

 tion suspected to contain this metal ; and afterward presenting 

 to it a small piece of nitrate of silver. It is far preferable, Mr. 

 Hume finds, to employ the latter salt in a solid form ; and he 

 recommends an angular piece, of the size of a pin's head, or 

 thereabouts, held at the surface of the fluid, on the point of a 

 knife. The existence of arsenic will be shown by a yellow 

 precipitate, which falls down in rather aburidant quantity. 

 Whenever arsenical mixtures are operated upon, which have qj-^^^j^j^j^ 

 but little contamination with foreign ingredients, this process it. 

 will, undoubtedly, succeed very well ; but if ever muriatic 

 acid be present, and this is always the case where materials 

 from the stomach are mixed with the fluids under experiment, 

 the test is then wholly useless, as a muriate of silver will be 

 immediately formed, and the yellow compound, said to be so 

 unequivocal in its indication of arsenic, of course be prevented 

 from appearing. Neither of the processes yet spoken of, 

 therefore, can receive that confidence, which ought to attach to 

 investigations of such high importance. This is not a mere spe- Tlie difficulties 

 culative difficulty, but was fully proved to me during a course P'^^.^'^'i '^y ex- 

 of experiments made some time ago, in consetjuence of a case 

 of poison which came under my notice ; and having, with the 

 assistance of my friend, Mr. James Oakes, devoted, at that 

 period, a good deal of attention to the subject, with a view, 

 if pos^ble, to supply the deficiency, we were led to the use of a dliTereat 

 X 2 two 



