PART L 
CHAPTER I. 
SCIENCES AUXILIARY TO GEOLOGY. 
Definition.— Physical Properties : Extension, Gravity, Impen- 
etrability, Divisibility, Porosity, Indestructibility, Inertia. 
Geology is that branch of physical science which 
treats of the constitution of the earth we inhabit. 
It not only has for its objects an investigation into 
the form and external characters of our globe, but 
also into the nature and relative position of the ma- 
terials which constitute its external crust, and the 
manner and order in which these materials have as- 
sumed their present position. Geology, therefore, 
embraces the science of Physical Geography ; but, 
before entering upon this branch of our subject, it 
will be proper to take a brief survey of those auxilia- 
ry sciences, some acquaintance with which is indis- 
pensable to a full understanding of what is to follow. 
The properties of material substances are of two 
kinds, physical and chemical ; the study of the for- 
mer comes under that department of science called 
Natural Philosophy^ the latter under that of Chemis- 
try. 
The physical properties are either general or sec- 
ondary ; those are called general which are com- 
mon to all bodies, while the secondary only belong 
to a few. Among the first may be enumerated Ex- 
tensio7i, Gravity, Impenetrability, Divisibility , Mobility, 
Porosity, Indestructibility, and Inertia, 
