20 CHEMISTRY OF THE EARTH. 



spheric air, and is essential to animal and vegetable 

 life. It is by the absorption of this gas that tlie 

 dark venous blood assumes a florid red colour in 

 the lungs ; and without this change, the functions 

 of the body would 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 tho 

 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, mto 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 perlectly 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 u 

 iiquid, vaj)Our, snow, or ice. 



