KINETIC THEORIES OF GRAVITATION. 219 



deed, if the atoms of matter are porous or penetrable totbe " ultramun- 

 dane corpuscles," the third condition will remain unsatisfied. 



This corpuscular system of course entirely' ignores the fourth condi- 

 tion of the problem, and its fundamental postulate stands in direct op- 

 l)osition to the fifth condition. It is certainly impossible, on auy quan- 

 titative assumption or numerical estimate whatever, to represent by 

 this scheme the earth's residual gravitation toward the sun during an 

 eclipse of the moon. 



Professor J. Clerk Maxwell, discussing the theory of Lesage, observes 

 that if the number of ultramundane corpuscles arrested by our earth is 

 by supposition much less than the number arrested by the suu, " the 

 proportiou of those which are stopped by a small body, say a one-pound 

 shot, must be smaller still in an enormous degree, because its thickness 

 is exceedingly small compared with that of the earth. Now the weight 

 of the ball, or its tendency toward the earth, is produced according to 

 this theory, by the excess of the impacts of the corpuscles which come 

 from above, over the impacts of those which come from below and have 

 passed through the earth. Either of these quantities is an exceedingly 

 small fraction of the momentum of the whole number of corpuscles 

 which pass through the ball in a second, and their difference is a small 

 fraction of either, and yet it is equivalent to the weight of a pound. 

 . . . . Now the energy of a moving system is half the product of its 

 momentum into its velocity. Hence the energy of the corpuscles which 

 by their impacts on the ball during one second, urge it toward the earth, 

 must be a number of foot-pounds equal to the number of feet over which 

 a corpuscle travels in a second, that is to say, not less than thousands 

 of millions. But this is only a small fraction of the energy of all the 

 impacts which the atoms of the ball receive from the innumerable streams 

 of corpuscles which fall upon it in all directions. Hence the rate at which 

 the energ3^ of the corpuscles is spent in order to maintain the gravitating 

 property of a single pound is at least millions of millions of foot-pounds 

 per second. What becomes of this enormous quantity of energy ? . . . . 

 The explanation of gravitation falls to the ground if the corpuscles are 

 like perfectly elastic spheres, and rebound with a velocity of separation 

 equal to that of approach. If on the other hand they rebound with a 

 smaller velocity, the effect of attraction between the bodies will no 

 doubt be produced ; but then we have to find what becomes of the 

 energy which the molecules have brought with them but have not car- 

 ried away. If auy appreciable fraction of this energy is communicated 

 to the body in the form of heat, the amount of heat so generated would 

 in a few seconds raise it, and in like manner the whole material universe, 

 to a white heat."* 



Hence the energy expended by the ultramundane corpuscles in giv- 

 ing motion to material masses must be so much abstracted from their 

 aggregate store of velocity ; and from the constantly-increasing num- 



*Encyclopa3dia Britauuica, uintli edition, 1875, article " Atom," vol. iii, pp. 4(3, 47. 



