368 THE CONSEBVATION OF FOKCE. 



The half assumption is, in my view of the matter, more dog- 

 matic and irrational than the whole, because it leaves it to be 

 understood that power can be created and destroyed almost at 

 pleasure. 



When the equivalents of the various forms of force, as far 

 as they are known, are considered, their differences appear 

 very great ; thus, a grain of water is known to have electric 

 relations equivalent to a very powerful flash of lightning. It 

 may therefore be supposed that a very large apparent amount 

 of the force causing the phenomena of gravitation, may be 

 the equivalent of a very small change in some unknown con- 

 dition of the bodies, whose attraction is varying, by change of 

 distance. For my own part, many considerations urge my 

 mind toward the idea of a cause of gravity, which is not res- 

 ident in the particles of matter merely, but constantly in 

 them, and all space. I have already put forth considerations 

 regarding gravity which partake of this idea,* and it seems 

 to have been unhesitatingly accepted by Newton.-f- 



There is one wonderful condition of matter, perhaps its 

 only true indication, namely, inertia ; but in relation to the 

 ordinary definition of gravity, it only adds to the difficulty. 

 For if we consider two particles of matter at a certain dis- 

 tance apart, attracting each other under the power of gravity, 

 and free to approach, they will approach ; and when at only 

 half the distance, each will have had stored up in it, because of 

 its inertia, a certain amount of mechanical force. This must 



* Proceedings of the Royal Institution, 1855, vol. ii., p. 10, etc. 



f " That gravity should be innate, inherent, and essential to matter, so 

 that one body may act upon another at a distance, through a vacuum, with- 

 out the mediation of any thing else, by and through which their action and 

 force may be conveyed from one to another, is to me so great an absurdity 

 that I believe no man who has in philosophical matters a competent fac- 

 ulty of thinking, can ever fall into it. Gravity must be caused by an agent, 

 acting constantly according to certain laws ; but whether this agent be ma- 

 terial or immaterial I have left to the consideration of my reader." See 

 Newton's TJdrd Letter to Bentley. 



