CHAPTEE III. 



THE RE-ACTIONS OF OEGANIC MATTEE ON FORCES. 



§ 17. Ee-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 



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