CHAPTEE III. 



THE RE-ACTIONS OF OBGANIC MATTER 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 one another; 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 themselves originate. But in organic 

 matter the re-arranged parts do not re-act in virtue of their 

 inertia only. They are so constituted that an incident force 

 usually sets up in them other actions which are much 

 more important. Indeed, what we may call the indirect re- 



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