1857.] . , The gravitating Force. 451 



tides of matter merely, but conjointly 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 Newtonf . 



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 distance apart, 

 attracting each other under the power of gravity and free to 

 approach, they will approach ; and when at only half the di- 

 stance, 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 conservation 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 producing 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 attractive 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 conser- 

 vation of force. It has a strict relation to gravity, as appears 

 by the proportionate amount of force which gravity can com- 

 municate to the inert body ; but it appears to have the same 

 strict relation to other forces acting at a distance, as those of 

 magnetism or electricity, when they are so applied by the tan- 



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



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, 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 think- 

 ing, 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 readers." See Newton's ' Third Letter 

 to Bentley.' 



