solution of nitrate of baryta occasioning a heavy white precipitate of
sulphate of baryta, which is insoluble in nitric acid. Selenic and
sulphurous acids, however, and also, as Mr. Alfred Taylor informs me he
has lately found, the fluo-silicic acid, are similarly acted on in all
respects. But selenic and fluo-silicic acids in all forms, and
sulphurous acid in a state of solution, are so seldom met with, being
known only in the laboratory of the scientific chemist, that they can
scarcely be considered sources of fallacy. Sulphuric acid may at once be
distinguished from sulphurous acid, by the latter possessing a peculiar
pungent odour. From the two other acids it may be distinguished by
collecting and drying the barytic precipitate, mixing this with
charcoal, converting it into sulphuret of barium by heating it in a
platinum spoon before the blowpipe, and then adding diluted muriatic
acid to the sulphuret, so as to disengage sulphuretted hydrogen
gas,—which again is easily known by its odour, or its property of
blackening paper dipped in solution of acetate of lead. A much more
important source of fallacy than these is the possible presence of a
bisulphate in solution, or a neutral sulphate along with any other free
acid; for these substances will present the same reactions with litmus
and barytic salts as free sulphuric acid itself. Much has been published
lately upon this point; but the difficulty has not yet been
satisfactorily overcome. It may be got rid of indeed by proving, that no
bisulphate or neutral sulphate is present. Their absence may be shown by
no solid residuum being left on evaporating the suspected fluid, or at
least no more than a mere haziness, owing to the sulphate of lead which
commercial sulphuric acid always contains in small quantity. Or as
Orfila suggests, we may establish their absence still better by
concentrating the fluid, and finding that neither carbonate of soda,
which would cause a precipitate with earthy or metallic bases, nor
chloride of platinum, which would do so with potash or ammonia in
combination, nor fluo-silicic acid, which precipitates soda salts, has
any effect when applied to separate portions of the subject of inquiry.
But suppose it appears in the course of these trials that one or more
bases are actually present, how is it to be settled whether the
sulphuric acid, indicated by litmus and a salt of baryta, is really free
or not? To this question I must reply, that no method has yet been
proposed, which is at once satisfactory and easily available. Mr. Alfred
Taylor proposes to concentrate the fluid, and agitate it with alcohol,
in the hope that the alcohol will remove sulphuric acid, and not a
sulphate, from the water.[234] But it removes sulphuric acid from a
bisulphate even when dry, and still more when a little water is present.
Orfila[235] proposes, in the case of sulphuric acid in vinegar,—where
there is both a vegetable acid and a neutral sulphate of lime,—to
concentrate to a sixth, and agitate the residuum with four times its
volume of sulphuric ether, in the expectation that this fluid will
remove the free acid alone, and separate it from sulphates. But
notwithstanding the authority of his name for the fact, pure ether will
not remove sulphuric acid from a watery fluid; and etherized alcohol,
which does remove it, takes it away also, like alcohol itself, from
bisulphates. These results I have observed in some careful trials made
along with Dr. Douglas Maclagan. I suspect, therefore, that where
sulphates or bisulphates do exist, there is no absolutely satisfactory
way of determining whether free sulphuric acid also co-exists, except by
a quantitative analysis, for ascertaining whether the amount of acid and
of bases corresponds with this supposition or not. And it is scarcely
necessary to add, that so operose a method is scarcely applicable to
ordinary medico-legal investigations.