CHAPTER III. 



THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 



17. RE-DISTRIBUTIONS of Matter, imply concomitant re 

 distributions of Motion. That which under one of its aspects 

 we contemplate as an alteration of arrangement among the 

 parts of a body, is, under a correlative aspect, an alteration 

 of arrangement among certain momenta whereby these parts 

 are impelled to their new positions. At the same time that 

 a force, acting differently on the different units of an aggre 

 gate, changes their relations to each other ; these units, re 

 acting differently on the different parts of the force, work 

 equivalent changes in the relations of these to one another. 

 Inseparably connected as they are, these two orders of phe 

 nomena are liable to be confounded together. It is very 

 needful, however, to distinguish between them. In the last 

 chapter, we took a rapid survey of the re-distributions which 

 forces produce in organic matter ; and here we must take a 

 like survey of the simultaneous re-distributions undergone by 

 the forces. 



At the outset we are met by a difficulty. The parts of an 

 inorganic mass undergoing re-arrangement by an incident 

 force, are, in most cases, passive do not complicate those 

 necessary re-actions that result from their inertia, by other 

 forces which they originate. But in organic matter, the 

 re-arranged parts do not re-act in virtue of their inertia only ; 

 they are so constituted that the incident force usually sets up 



