[p. 211], the flame yields a dark brownish-black, obscurely shining
crust on a surface of porcelain held across it, and a white crystalline
powder if the porcelain be held just above the flame. The dark crust is
antimony, the white one its oxide. The former has only a distant
resemblance to the brilliant stain of arsenic, notwithstanding all that
has been said of their similarity. It is well, however, to use some
other test for distinguishing the two metals besides their appearance;
and the most convenient is a solution of chloride of lime, which
instantly makes an arsenical crust disappear, but does not affect an
antimonial one.
Tartar-emetic, like the soluble salts of mercury and copper, is
decomposed by various organic principles. All vegetable substances that
contain a considerable quantity of tannin have this effect; of which an
example has been already mentioned in the action of infusion of galls.
Decoctions of cinchona-bark decompose it still more effectually. The
animal principles do not act on tartar-emetic, with the exception of
milk, which is slightly coagulated by a concentrated solution. Many
vegetable and animal substances, though they do not decompose it, alter
the operation of the fluid tests. Thus tea, though it does not effect
any distinct decomposition of the salt, will prevent the action of
gall-infusion; and French wine gives a violet tint to the precipitates
with that test and with acids.[1130] Hydrosulphuric acid, however, acts
under all circumstances, and always characteristically, whatever the
colour of the fluid may be. Dr. Turner found that when transmitted
through a diluted solution in tea, porter, broth, and milk, with certain
precautions to be mentioned presently, he procured a precipitate which
either showed its proper colour at once, or did so at the margin of the
filter on which it was collected.[1131]
The circumstances now referred to render it necessary to resort to other
means, besides the simple application of liquid reagents, for the
purpose of detecting tartar-emetic in complex organic mixtures. This
subject has been ably investigated, first by Dr. Turner,[1132] and
afterwards by Professor Orfila.[1133] The result of the researches of
both seems to me to be that the most convenient method yet proposed is
the following.
_Process for Tartar-emetic in Organic Mixtures._—If the subject of
analysis be not already liquid enough, add distilled water. Then
acidulate with a little hydrochloric and tartaric acids; the former of
which throws down some animal principles, while the latter dissolves
readily all precipitates formed with tartar-emetic by reagents or
organic principles except the sulphuret. Filter the product.