20 CHEMISTRY OF THE EARTH. 
spheric air, and is essential to animal and vegetable 
life. It is by the absorption of tbis gas that the 
dark venous blood assumes a florid red colour in 
the lungs ; and without this change, the functions 
of the body v^ould soon languish, and death speed- 
ily ensue. The quantity of oxygen which enters 
into the constitution of our globe is enormously 
great. It forms 20 per cent, of the volume of the 
atmosphere ; a third part, by measure, of the gases 
composing water; and rocks embrace an immense 
amount besides. Silica has been computed to con- 
stitute 45 per cent, of the mineral crust of the earth ; 
and silica consists of 48 parts silicium and 51 parts 
of oxygen. 
When we consider, then, the vast amount of oxy- 
gen contained in the waters of the ocean, in the at- 
mosphere, and in the rocks, we shall perceive that 
it is one of the most abundant elements which enters 
into the chemical constitution of the globe. 
Hydrogen^ sometimes called inflammable air, is the 
the atmospheric air is understood as the standard of comparison. 
Gases are produced in immense quantities in the interior of the 
earth by the influence of internal heat, and there can be no 
doubt that many earthquakes and volcanic eruptions are caused 
by the sudden irruption of water upon internal fires. This may 
be illustrated by the explosion of powder, which is a mixture 
of charcoal, saltpetre, and sulphur. These substances are sud- 
denly decomposed, and gases formed by a combination of their 
elements, the production of which causes a violent concussion 
in the atmosphere productive of sound. It is now ascertained 
that many of the gases may be reduced, by powerful compres- 
sion, into liquids ; as, for example, carbonic acid gas, which, 
under the enormous pressure of 36 atmospheres, is first reduced 
to a state of liquefaction, and then to a solid, while, at the same 
time, a degree of cold is produced equal to — 136 degrees of Fah- 
renheit. In this form it is perfectly white, like snow, and of 
a soft and spongy texture. Quicksilver, when brought in con- 
tact with it, becomes solid, and can be cut and hammered out 
like lead. If applied to the skin, it speedily destroys the part, 
indeed as rapidly as a red-hot iron. We see, then, that one of 
the gases may exist either in an aeriform state, or in that of a 
Jiquid, vapour, snow, or ice. 
