390 Dr Christison on the Detection of 
the amoniacal nitrate of silver, and sulphuretted hydrogen, are 
almost or absolutely useless. He next proves, that no advan- 
tage is to be derived from the plans which have been proposed 
for restoring the true action of the tests, by destroying the co- 
lour of the fluids. These are two in number ; the one advanced 
by Grfila in 1821 ; the other by Mr Phillips in January last. 
The former chemist proposes to destroy the colours by chlorine ; 
the latter by digestion with animal charcoal. But Dr Christi- 
son finds, that, after the action of chlorine, the colour is seldom 
altogether, or sufficiently destroyed ; that the process does not 
take from the fluid the power, it often possesses, of retaining the 
arsenical precipitate in solution ; and that, in some decolorised 
fluids, containing no arsenic, some of the tests cause precipitates 
very similar to those produced in pure arsenical solutions. He 
likewise finds, that the process by digestion in charcoal is suffi- 
cient, because, if the solution is not very strong, the charcoal 
removes all, or nearly all, the arsenic, as well as the colouring 
matter ; and, if the solution be strong, it does not always lose 
its property of retaining the arsenical precipitates dissolved. 
He then proceeds to examine the processes which have been re- 
commended by Hose and Rapp for discovering arsenic, when 
intimately mingled with the animal textures, and which might 
also be applied to the residue by evaporation of mixed fluids, in 
which the common tests do not act characteristically. These 
processes it is unnecessary to mention. They are founded on 
the possibility of arsenic so combining with animal matters, as to 
resist the solvent power of boiling water. But Dr Christison 
has found, that, after careful digestion in water, no arsenic can 
be detected by either process ; and, farther, that the process of 
Rapp is otherwise insufficient, when the quantity of arsenic is 
small, in which circumstance alone, the ordinary means of ana- 
lysis are inadequate. 
In the last place, he describes the method which he has found 
easiest of execution, and most generally applicable, for detecting 
arsenic dissolved in mixed fluids, or mingled with such solids as 
are incapable of forming with it an insoluble compound. Mi- 
nute directions are given for the sake of the inexperienced ; but 
w r e shall notice only the essential parts, and the result of his ex- 
perience with respect to the delicacy of the method. Having 
observed, that the sulpheretted hydrogen, though it seldom acts 
