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, 



