tillation from sulphat of
iron, in which sulphuric acid and oxyd of iron are combined, according
to the process described by Basil Valentine in the fifteenth century;
but, in modern times, it is procured more oeconomically by the
combustion of sulphur in proper vessels. Both to facilitate the
combustion, and to assist the oxygenation of the sulphur, a little
powdered saltpetre, nitrat of potash, is mixed with it; the nitre is
decomposed, and gives out its oxygen to the sulphur, which contributes
to its conversion into acid. Notwithstanding this addition, the sulphur
will only continue to burn in close vessels for a limited time; the
combination ceases, because the oxygen is exhausted, and the air of the
vessels reduced almost to pure azotic gas, and because the acid itself
remains long in the state of vapour, and hinders the progress of
combustion.

In the manufactories for making sulphuric acid in the large way, the
mixture of nitre and sulphur is burnt in large close built chambers
lined with lead, having a little water at the bottom for facilitating
the condensation of the vapours. Afterwards, by distillation in large
retorts with a gentle heat, the water passes over, slightly impregnated
with acid, and the sulphuric acid remains behind in a concentrated
state. It is then pellucid, without any flavour, and nearly double the
weight of an equal bulk of water. This process would be greatly
facilitated, and the combustion much prolonged, by introducing fresh air
into the chambers, by means of several pairs of bellows directed towards
the flame of the sulphur, and by allowing the nitrous gas to escape
through long serpentine canals, in contact with water, to absorb any
sulphuric or sulphurous acid gas it might contain.

By one experiment, Mr Berthollet found that 69 parts of sulphur in
combustion, united with 31 parts of oxygen, to form 100 parts of
sulphuric acid; and, by another experiment, made in a different manner,
he calculates that 100 parts of sulphuric acid consists of 72 parts
sulphur, combined with 28 parts of oxygen, all by weight.

This acid, in common with every other, can only dissolve metals when
they have been previously oxydated; but most of the metals are capable
of decomposing a part of the acid, so as to carry off a sufficient
quantity of oxygen, to render themselves soluble in the part of the acid
which remains undecomposed. This happens with silver, mercury, iron, and
zinc, in boiling concentrate