

NATURAL PHILOSOPHY. 183 



equivalent of a very small change in some unknown condition 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 resident 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 

 Ke \vton.t 



There is one wonderful condition of matter, perhaps its only true indica- 

 tion, 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 distance 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 be due to the force exerted, and, if the con- 

 servation principle be true, must have consumed an equivalent proportion 

 of the cause of attraction ; and yet, according to the definition of gravity, the 

 attractive force is not diminished thereby, but increased four-fold, the force 

 growing up within itself the more rapidly, the more it is occupied in produc- 

 ing other force. On the other hand, if mechanical force from without be 

 used to separate the particles to twice their distance, this force is not stored 

 up in momentum or by inertia, but disappears ; and three fourths of the at- 

 tractive force at the first distance disappears with it. How can this be ? 



We know not the physical condition or action from which inertia results ; 

 but inertia is always a pure case of the conservation of force. It has a strict 

 relation to gravity, as appears by the proportionate amount of the force 

 which gravity can communicate to the inert body ; but it appears to have the 

 same strict relation to other forces acting at a distance as those of magnetism. 



o o 



or electricity, when they are so applied by the tangential balance as to act 

 independent of the gravitating force. It has the like strict relation to force 

 communicated by impact, pull, or in any other way. It enables a body to 

 take up and conserve a given amount of force until that force is transferred 

 to other bodies, or changed into an equivalent of some other form ; that is 

 all that we perceive in it ; and we cannot find a more striking instance 

 amongst natural, or possible phenomena of the necessity of the conservation 

 of force as a law of nature ; or one more in contrast with the assumed vari- 

 able condition of the gravitating force supposed to reside in the particles of 

 matter. 



Even gravity itself furnishes the strictest proof of the conservation of force 



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



t "That gravity should be innate, inherent, and essential to matter, so that one 

 body may act upon another at a distance, through a I'acuum, without the mediation 

 of anything 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 faculty 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 material or immaterial I have left to the consideration of 

 my reader." See Newton's Third Letter to Bentley. 



