ences an impetus on all sides; but if we take two bodies each screens the other and they are pushed together. Notice the force is one of pressure and not of tension. At first sight the impulse due to the impact would seem to be proportional to the effective area whereas according to Newton's law it ought to be proportional to the mass; but when we compare the diameter of a molecule with the distance between molecules, we see that only a small portion of the particles are arrested and that the number arrested is proportional to the number of molecules in the body (the mass).
The objections to Le Sage's theory are almost too numerous to mention. First and foremost, the enormous speed at which these corpuscles must travel not to resist planetary motion involves an enormous supply of energy from a source outside our universe. On this theory the source of gravitation is ultramundane. Again if these corpuscles are elastic there would be no screening action on the part of a body as the corpuscles would carry away their energy in reflexion. If the corpuscles are inelastic, bodies ought to increase in size. As the corpuscles transfer their momentum to bodies they lose kinetic energy, and according to Maxwell the loss sufficient to account for gravitation if converted into heat would keep the body white hot. Sir J. J. Thomson has shown that it is not necessary to suppose the energy is transformed into heat. In place of heat rays he suggests that the particles might give rise to a very penetrating radiation just as the cathode particles are supposed to give rise to the short ether-pulse known as Röntgen rays.
The fact that a physicist of Kelvin's rank has tried to patch up a theory that is so superficial shows how hard put to we are when we attempt to explain gravitational attraction.
About thirty years ago Zöllner explained gravitation on the assumption that the molecules carry positive and negative charges and the attraction between two unlike charges exceeds the repulsion between like charges. Lorentz, assuming a neutral body to be an assemblage of positive and negative electrons, has used the same hypothesis.
At Cambridge University during the fall term of 1908, Sir J. J. Thomson gave a course of lectures on "Ether and Matter" and in that course he devoted about three lectures to gravitation. Take two charged plates and in place of drawing the resultant Faraday lines of force (a Faraday line is a line beginning on a unit positive charge and terminating on a unit negative charge) consider the components (see Fig. 1).