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XX. Mr. Hume's Remarks on his Test for Arsenic, 



To Mr. Tilloch. 



Sir, JL he great importance of a sure method to detect 

 arsenic, whether the question be considered judicially or 

 chemically, induces me once more to address you upon 

 this subject, particularly as the test, originally discovered 

 by myself, is now clearly demonstrated to surpass all others 

 in precision and efficacy ; for, such is its power that, by 

 proper management, it will prove the existence of white 

 arsenic, even if dissolved in more ih&n four hundred thou- 

 sand times its weight of water, when, probably, in such a 

 rare state of dilution, this dreadful body ceases to be a di- 

 rect poison. 



In my letter to you, inserted in the xxxiiid volume of 

 your excellent Magazine, and in two other letters published 

 afterwards in the " Medical and Physical Journal," dated 

 May and October 1810, 1 have, in general terms, described 

 the best process for discovering the arsenic to be, that of 

 combining it with silver. To effect this, I advised that the 

 arsenic should be nearly saturated v/ith any alkali; and 

 that nitrate of silver, the common lunar caustic, should then 

 be applied to the surface of the solution in which the poi- 

 son is suspected to exist ; — if a bright yellow colour appear 

 on presenting the nitrate of silver, we may conclude with- 

 out reserve, that some of this poison is in the mixture un- 

 der examination. 



By the three letters just quoted, it will be seen that this 

 plan is not confined to any particular alkali, as the sole use 

 of such addition is merely to neutralize the arsenic, so that 

 the nitrate, or indeed any other salt of silver, may be more 

 readily decomposed, and beyond this point the quantity of 

 alkali should not pass. 



But, lest any one disapprove of this scheme, urging the 

 difficulty of assigning the (luarilily of alkali requisite for the 

 point of saturation, I shall give, in the present communica- 

 tion, another prescription, which has so completely suc- 

 ceeded with mc that, 1 presume, it must now supersede all 

 other tests for arsenic, and become the standard to future 

 operators. The name which I shall In-g to olTer for this test 

 'M^ainmoiiiaco-nilrate of silver ; and though the proportion of 

 the water to the (dher ingredients is of little consequence, 

 itill the li(iuor when (iuibhcd will always be sufficiently 

 uniform, and quite free from any imperfection whatever, 

 provided the articles llicmselves be pure. 



Dissolve 



