CHAPTEE III 



THE PRINCIPLES OF ENERGY 



So far our description of the animal body has been that of a 

 mechanism which can be actuated, or made to "go." How it is 

 actuated — that is, what are the sources of its energy, and how 

 these sources are utiHsed — is the subject of the following chapter. 

 Meanwhile, however, something must be said about the general 

 principles of energy in so far as they concern us in our study of 

 life. First, then, we ought to consider what is meant by 



The Nature of a Material Body. 



A material thing or body is something that is heavy; that has 

 shape, or occupies space; that coheres and is dense in varying 

 degrees; that has heat, also in varying degrees; that has texture, 

 lustre, colour, smell, and taste. In short, a material body is a 

 massive substance which has physical " properties." 



Coherence. — Material bodies may be solid, liquid, or gaseous. 



'When they are solid they can be disintegrated by mechanical 

 means — that is, they can be crushed, broken, powdered, filed, 

 etc. — and therefore they must consist of smaller parts that cohere 

 together, but which can, nevertheless, be separated. There 

 appears to be a limit (in the practical sense) to the degree to 

 which a solid material can be pulverised, though we can reduce 

 it to exceedingly fine particles. By-and-by, however, our 

 mechanical means of disintegration fail to make the particles any 

 finer, but in imagination we can still divide them. 



The chemist can show that the finest particles to which a body 

 can be reduced are still aggregates of molecules, and thus he 



' regards the latter as the very finest particles to which a material 

 body can be reduced without losing its specific properties. 

 Molecules are therefore the ultimate particles of which bodies 

 are made up, and they cohere together more or less strongly, or 

 not at all. When the degree of cohesion is such that the body 

 preserves its shape irrespective of anything that contains it, we 



, say that it is solid ; but when the molecules still cohere, but slip on 

 each other, we call the body viscous (as in the cases of pitch or 



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