50 
PHYSICAL GEOGRAPHY. 
bleaching cotton, linen, paper, &c. When melted 
with borax, it always gives a purple tinge, by which 
it may be distinguished from iron and other substan- 
ces. The other metals occur so rarely, and are of 
so little importance, that we shall pass them over ; 
referring the reader, however, to Cleaveland, Corn- 
stock, Dana, and other authors who have treated of 
mineralogy, for any information in relation to them. 
CHAPTER IV. 
PHYSICAL GEOGRAPHY OF THE EARTH. 
Shape of the Earth. — Mean Density. — Ocean — Mean depth.— 
Saline Contents. — Mediterranean. — Temperature of Earth. — 
Reason for Supposing it has undergone a Change. — Central 
Heat. — Facts to Support such a Theory. — Influence of Chmate 
on the Animal and Ve^table Kingdom. — Height of Mount- 
ains. — Highest Land in Asia. — In Europe. — In America.— 
Shape of Hills and Mountains. 
The figure of the earth has been compared to that 
of an orange, being a flattened spheroid. The equa- 
torial diameter is about 7924 miles. 
The polar axis 7898 " 
Difference 26 " 
It can be demonstrated that a fluid body, possess- 
ed of rotatory motion in space, would assume a sim- 
ilar shape. 
The mean density of the earth has been variously 
estimated, but, from experiments performed by dif- 
ferent philosophers, it is now believed to be about 
five times greater than that of water, and, conse- 
quently, double that of the mineral crust of our 
globe. 
