This was formerly called “Oil of Vitriol,” because it has an oily
consistence, and was originally distilled from green vitriol.
Sulphuric acid is one of the most useful chemical agents known;
scarcely a process in chemical manufacture can be performed without
its assistance. Sulphuric acid is colorless, and very heavy, being
nearly double that of water (the proportion is 1·842, while water is
1,000); it is powerfully acid, even when largely diluted with water,
and during this mixture with water gives out great heat, a mixture
of equal parts will become hotter than boiling water; if it be mixed,
however, with snow instead of water, it becomes extremely cold (below
zero).
Sulphuric acid is made by burning sulphur in a furnace, and causing
the vapours to pass into a large chamber lined with lead, and having
some water at the bottom; into this chamber the vapours arising from
nitre mixed with sulphuric acid are also admitted, together with
air and a jet of steam. These arrangements require to be properly
and nicely regulated, and it was only by study of the complicated
changes which take place between these gases (sulphurous acid, nitric
oxide, and atmospheric air), that this arrangement has been devised
and adopted, a great part of the product having been formerly wasted
in a more clumsy mode of preparation. The sulphuric acid condensed
in the water at the bottom of the leaden chamber is too weak for
use, and is concentrated by evaporating the water from it; for this
purpose it is placed in shallow leaden pans placed on the bars of a
furnace, and finally distilled in glass retorts, or retorts of iron
lined with platinum. Sulphuric acid has such a powerful attraction
for water, that an open vessel half-full of strong acid placed in
a damp situation will attract enough water from the atmosphere to
cause it to be quite full before long. This power of attracting water
has been taken advantage of to procure ice in those places where it
is not to be had naturally, as India, &c. If a vessel of water be
placed under the receiver of an air-pump, and the receiver exhausted
of air, the vapour of the water will speedily fill it, taking the
place of the air, and so stop any further evaporation; but if another
vessel containing some sulphuric acid be placed also in the receiver,
the acid will absorb the vapour of water as fast as it is formed,
and this rapid evaporation continuing produces such cold, that the
remaining water is shortly frozen. Iron and zinc dissolve rapidly
in diluted sulphuric acid, giving off abundance of hydrogen gas, and
this was the way this gas was formerly produced for the inflation of
balloons, but the common coal-gas being easily obtainable in almost
any quantity in all towns, it is now used for that purpose instead
of hydrogen gas. Sulphuric acid (chemically considered) consists
of 1 equivalent of sulphur with 3 of oxygen, and 1 of water. Pure
sulphuric acid (without water) is in the form of fine crystals, much
resembling snow, which, on exposure to air containing the slightest
quantity of moisture, absorbs it, and becomes converted into the
ordinary sulphuric acid.
If this acid (even when greatly diluted) be spilt on cotton or
linen it destroys it, producing a hole; this is owing to the acid
converting the fibre into sugar. A proposition was once made to
produce sugar from this source, but linen and cotton rags are in
too great demand for paper-making to allow of its being done.