CHEMISTRY OF THE EARTH. 19 
these four substances. Now, however, it is known 
that there are more than fifty elements, of which 
most are metals. By an element is understood a 
body which has not been decomposed, and is there- 
fore believed to consist of one substance only, al- 
though it is possible that it may consist of more. 
The principal elements or substances which en- 
ter into the composition of the earth's surface are 
14 in number, and may be classed in the following 
order, according to the respective importance of 
each. 
Simple Non- Metallic Substances, 
1. Oxygen. 
2. Hydrogen. 
3. Nitrogen. 
4. Carbon. 
5. Sulphur. 
6. Chlorine. 
7. Fluorine. 
8. Phosphorus. 
Metallic Bases of the Alkalies and Earths. 
1. Silicium. 4. Sodium. 
2. Aluminium. 5. Magnesium. 
3. Potassium. 6. Calcium. 
Fourteen simple substances, then, by their various 
combinations, form by far the largest amount of all 
the matter, whether gaseous, liquid, or solid, organ- 
ic or inorganic, which is known to exist on the 
surface of the earth. 
Oxygen is an invisible gas,* heavier than atmo- 
* A gas is an invisible, transparent, elastic fluid, which re- 
mains in an aeriform state, and therefore differs from a vapoury 
which, by cooling, returns to its original liquid or solid form. 
Thus, water, mercury, sulphur, &c., can be converted into va- 
pour by heat ; but they soon resume their solid form upon cool- 
ing. In this manner sulphur is sublimed, and collected for use 
in the arts and in medicine ; and it also collects in large quan- 
tities on the craters of volcanoes and on the borders of sulphu- 
rous springs. When we speak of the specific gravity of a gas, 
