1, 2, 3, 4, 5 " sulphurous acid and
3, 5, 7, 9, 11 " sulphureted hydrogen.

Thus we find that:

Carbonate of lime, 2CaCO_{3}, } { 2(CaSO.H_{2}O) (gypsum),
Sulphurous acid, 2SO_{2}, and } yield { 4S (sulphur), and
Sulphureted hydrogen, 4H_{2}S, } { C_{2}H_{4}, which is
{ _ethylene_.

And that:

Carbonate of lime, CaCO_{3} } { (CaSO_{4}.H_{2}O) (gypsum),
Sulphurous acid, SO_{2}, and } yield { 3S (sulphur) and
Sulphureted hydrogen, 3H_{2}S, } { CH4, which is marsh gas.

So that these and all their homologues, in fact petroleum in all its
varieties, would be produced in nature by the action of volcanic gases
on limestone.

But much the most abundant of the volcanic gases appear at the surface
as steam, and petroleum seems to have been more usually produced
without sulphurous acid, and with part of the sulphureted hydrogen
(H_{2}S) replaced by H_{2}O (steam) or H_{2}O_{2} (peroxide of
hydrogen), which is the product that results from the combination of
sulphureted hydrogen and sulphurous acid:

(H_{2}S + SO_{2} == H_{2}O_{2} + 2S).

It is a powerful oxidizing agent, and it converts sulphurous into