SECT. XXXIV. GRAVITY. 355 



liave been within the conception of Newton himself when he 

 gave utterance to the law ; but that the totality of a force can 

 be employed according to that law I do not beliove either in 

 relation to gravitation, or electricity, or magnetism, or any other 

 supposed form of power. That there should be a power of 

 gravitation existing by itself, having no relation to the other 

 natural powers, and no respect to the law of the conservation of 

 force, is as little likely as that there should be a principle of 

 levity as well as gravity. Gravity may be only the residual part 

 of the other forces of nature, as Mossotti has tried to show ; but 

 that it should fall out from the law of all other forces, and should 

 be outside the reach either of farther experiment or philosophical 

 conclusions, is not probable. So we must strive to learn more of 

 this outstanding power, and endeavour to avoid any definition 

 of it which is incompatible with the principles of force generally, 

 for all the phenomena of nature lead us to believe that the great 

 and governing law is one. Thus gravitation can only be con- 

 sidered as part of a more general force whose law has yet to be 

 discovered. 



The definition of the gravitating force immediately suggests 

 the question of how it is transmitted ; the full force of that 

 question was felt by Newton himself when, in his third letter to 

 Bentley, he wrote, "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 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 philosophic 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 readers." 



Since Newton's time the continual decrease in the periodic 

 times of the comets belonging to our system, and the undulatory 

 theory of light and heat, have proved the existence of an extremely 

 rare elastic medium filling space even to the most distant regions 

 of which we are cognizant. But, rare as it may be, it has inertia 

 enough to resist the motion of comets, and therefore must be 

 material, whether considered to be ether or, according to Mr. 

 Grove, the highly attenuated atmospheres of the celestial bodies. 



