apt to occur in _sulphuric acid_, when that substance is prepared with
pyritic sulphur, which commonly contains some sulphuret of arsenic; and
it has actually been found in abundance in the acid by various
experimentalists, and in England for the first time by Dr. Rees.[535] It
may be detected by transmitting hydrosulphuric acid gas through the
diluted acid; and it may be effectually removed in the same way,[536]
the acid being afterwards filtered in a funnel whose throat is
filled with asbestus, and the excess of gas being expelled by
heat.—_Hydrochloric acid_ may contain arsenic, because it may have been
prepared with an arsenicated sulphuric acid. The impurity may be
detected and removed in the same way as in that substance. Nitric acid
seems not apt to be similarly adulterated;[537] but it may be tested by
Marsh’s process, after neutralizing the acid with potash, and adding
more sulphuric acid than is required to decompose the nitre thus formed.
_Zinc_ occasionally contains a little arsenic, which will be evolved in
Marsh’s process. Dr. Clark of Aberdeen says zinc is scarcely ever free
of a trace of arsenic; and it has been occasionally detected by others.
Orfila, however, very seldom found so much as to be discoverable by
Marsh’s test applied continuously for a great length of time.[538] A
committee of the French Institute came to the same conclusion.[539] M.
Jaquelain, acting under the directions of Professor Dumas, could not
detect an atom in any French specimen of zinc, or its carbonate or
silicated oxide, as met with in commerce.[540] Lastly, Mr. Brett
satisfied himself that no British or foreign zinc he could obtain
indicated the presence of arsenic by a process capable of detecting a
5000th of that metal in zinc.[541] It is an obvious inference from all
these inquiries that no difficulty can be experienced in obtaining zinc
so pure as to exhibit not a trace of arsenic by Marsh’s method. Neither
is there any difficulty in obtaining sulphuric, muriatic, and nitric
acid free of that adulteration.
But at the same time it is equally obvious, that in medico-legal
analyses, unless the reagents used be previously known to be free of
arsenic, they ought invariably to be subjected in the first instance to
the process, whatever it may be, which the analyst proposes to employ
for detecting arsenic in a suspected substance.