610 ON DETECTING THE 
however high his reputation as a chemical analyst, 
or as a toxicologist, has a right to be satisfied that 
any metallic looking substance is arsenic, which, 
by the apparatus in question, he may have sepa- 
rated from matter suspected to contain it, unless 
the quantity is so great that he can verify his 
suspicions, not only by ascertaining the tempera- 
ture at which it is volatile, but by the application 
of other indubitable tests. 
Before concluding, I must not neglect to men- 
tion the great necessity which exists, in the appli- 
eation of ‘‘Marsh’s Test,” previously to examine 
with the utmost care the purity of the articles to be 
used. Itis requisite to see that the apparatus is en- 
tirely free from arsenic; and that not only the zinc 
is pure, but particularly the sulphuric acid ; for, 
at present, the sulphuric acid of commerce, as 
ordinarily produced from pyrites, is strongly im- 
pregnated with arsenious acid.* And from facts 
which have come under my observation, I may 
add that hydro-chloric acid (muriatic acid) pro- 
duced through the agency of such sulphuric acid 
is also impregnated with arsenic. I have not yet 
* Since this paper was read, I have found 1000 grains, by 
weight, of a commercial sample of concentrated sulphuric 
acid, to contain 54 grains of arsenious acid. 
