is now generally known, that the only chemical antidote for arsenic is
the hydrated sesquioxide of iron. But this substance appears
occasionally to contain a little arsenic, obviously derived from the
compound of iron whence the oxide is prepared.[544] Such an adulteration
must be rare in what is prepared by the ordinary processes, according to
which the oxide of arsenic ought to remain in solution. The only
effectual mode, however, of guarding against this source of error, when
the antidote has been administered, is to examine a portion of the stock
whence the patient was supplied, by dissolving it in an excess of
sulphuric acid, and subjecting it to Marsh’s test.