QUALITIES. A liquid emitting fumes of a flame-red colour, and of a very
pungent and remarkable odour. The acid is either blue, green,
straw-coloured, clear orange yellow, according to the proportion of
nitrous acid gas[353] with which it is charged. CHEMICAL COMPOSITION.
This acid is improperly denominated _Nitrous_, for it is nitric acid,
holding nitrous acid gas loosely combined; by dilution this last
constituent is disengaged, and the acid, after passing through a
succession of different colours, becomes pure nitric acid; the
application of a gentle heat effects the same changes.
ACIDUM SULPHURICUM. L.E.D. Sulphuric Acid.
_Oil of Vitriol, Vitriolic Acid._
QUALITIES. _Form_, a thick liquid of an oily consistence, specific
gravity 1·85; a fluid-ounce weighs a fraction of a grain more than
fourteen drachms. _Colour_, none, but it acquires a brown tinge from the
smallest portion of carbonaceous matter; mere exposure to the air is
sufficient for this purpose, in consequence of the acid disorganizing
and carbonating the vegetable and animal matter suspended in the
atmosphere; it is therefore evident that bottles in which it is
preserved ought not to have stoppers of cork, but those of glass.
CHEMICAL COMPOSITION. Like the other mineral acids, it has never been
obtained in an insulated state without water; according to Davy, the
composition of the strongest acid may be thus expressed: sulphur 30,
oxygen 45, water 17. It has a very powerful affinity for water, and
produces when mixed with it a very considerable heat; exposed to the
atmosphere it imbibes at least seven times its own weight of water, and
so rapidly as to double its weight in a month; when of the specific
gravity 1·85, it rises in vapour at about 550°, and distils unaltered,
whereas weaker acids lose water by being boiled, and are brought to that
degree of concentration; when diluted with 12 or 13 per cent. of water,
an acid results of the specific gravity 1·780, and in this state of
dilution it boils at 435°, and freezes sooner than water; a knowledge of
this curious fact suggests to the prudent chemist an important
precaution; Mr. Parkes, in his Chemical Essays, vol. ii. relates the
occurrence of a terrible accident which happened in consequence of this
circumstance not having been attended to.—“Carboy after carboy burst by
the expansion of the acid in the act of freezing, and had not the packed
carboys that remained been immediately immersed in tepid water, not a
single one would have escaped the general wreck.”
ADULTERATIONS. The ordinary acid of the shops contains in general 3 or 4
per cent. of saline matter, which consists of about two-thirds of
sulphate of potass, and one-third of sulphate of lead. Dr. Ure observes,
that even more is occasionally found, in consequence of the employment
of nitre to remove the brown colour given to the acid by carbonaceous
matter; the amount of adulteration, he observes, may be readily
determined by evaporating a definite weight of the acid in a small
capsule of platinum; these impurities however in a medical point of view
are immaterial, since they are at once separated by dilution, but in a
commercial sense they deserve attention, as their presence considerably
increases the specific gravity of the acid. Dr. Ure is of opinion that
genuine commercial acid should never exceed 1·8485, and that any density
beyond this is the effect of saline combination. _Journal of Science and
the Arts, No. 7._