476 



The explanation of gravitation, therefore, falls to the ground if the cor- 

 uosouW are like perfectly elastic spheres, and rebound with a velocity of 

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

 nailer 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 carried away. 



If any 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 

 HUM it, and in like manner the whole material universe, to a white heat. 



It lias been suggested by Sir W. Thomson that the corpuscules may be 

 m constructed as to carry off their energy with them, provided that part of 

 their kinetic energy is transformed, during impact, from energy of translation 

 to energy of rotation or vibration. For this purpose the corpuscules must be 

 material systems, not mere points. Thomson suggests that they are vortex 

 atoms, wliich are set into a state of vibration at impact, and go off with a 

 smaller velocity of translation, but in a state of violent vibration. He has also 

 suggested the possibility of the vortex corpuscule regaining its swiftness and 

 losing part of its vibratory agitation by communion with its kindred cor- 

 puscules in infinite space. 



We have devoted more space to this theory than it seems to deserve, 

 because it is ingenious, and because it is the only theory of the cause of 

 gravitation which has been so far developed as to be capable of being attacked 

 and defended. It does not appear to us that it can account for the tem- 

 |H-niture of bodies remaining moderate while their atoms are exposed to the 

 tamhardment. The temperature of bodies must tend to approach that at 

 which the average kinetic energy of a molecule of the body would be equal 

 to the average kinetic energy of an ultramundane corpuscule. 



Now, suppose a plane surface to exist which stops all the corpuscules. 

 The pressure on this plane will be p = NMiiP where M is the mass of a cor- 

 puscule, N the number in unit of volume, and u its velocity normal to the 

 plane. Now, we know that the very greatest pressure existing in the universe 

 must be much less than the pressure p, which would be exerted against a body 

 which stops all the corpuscules. We are also tolerably certain that N, the 

 number of corpuscules which are at any one time within unit of volume, is 

 small compared with the value of N for the molecules of ordinary bodies. 

 Hence, Jfu a must be enormous compared with the corresponding quantity for 



