THEORY OF MATTER. 47 



imagined not resoluble en derntire analyse, into local motion 

 to the assertion that mechanical force is the only agency to be 

 recognised in the material universe is altogether illusory. For 

 matter may act on matter in a manner wholly distinct from 

 force, and yet this kind of action shall, ultimately and indi- 

 rectly, manifest itself in modifications of local motion. Further- 

 more, if for an instant we call this kind of action (force) 2 , we 

 shall at once be led to recognise a hypothetically possible mode 

 of action of matter on matter which in accordance with analogy 

 we shall call (force) 3 , which consists in the power of modifying 

 (force) 2 . And so on, sine limite. 



10. If we compare the language in which the relation 

 between mechanical force and chemical affinity is commonly 

 spoken of, we shall I think perceive its analogy with that which 

 I have used in describing the mode of action which we have 

 called (force) 2 . Its chemical affinity is spoken of as something 

 which suspends or modifies the action of force, as something 

 distinct from it, but which yet interferes with its effects. Or 

 again, if in physiological writings we observe the manner in 

 which vital action* is described we recognise, or seem at least 

 to do so, the possibility of referring its effects to that mode of 

 action which we have called (force) 3 . I do not however wish to 

 lay much stress on these similarities, because I think the kind 

 of reasoning we have pursued shows more satisfactorily than they 

 can do, that if chemical affinity and vital action are not resoluble 

 into force, they must be referred to some of the modes of action 

 we have pointed out. 



It would be useless to remark on the many points of specu- 

 lation which here present themselves. The expansion of bodies 

 by heat may however be particularly mentioned, because not- 

 withstanding what has been learnt with relation to the theory of 

 heat, nothing like a mechanical explanation of this phenomenon 

 has as yet been discovered. It seems to depend not on the 

 introduction of new mechanical forces, but on a modification of 

 those which already exist ; such modification, in cases of ordi- 

 nary conduction, being propagated from one part of the body to 

 that which is next it. It is easy to conceive that by an altera- 



* I am, of course, not to be understood as suggesting a materialistic explana- 

 tion of phenomena of thought or volition. 



