210 SPECULATIVE SCIENCE 



of Lucretius. Lesage asked you to conceive two solid bodies in 

 space, say the earth and sun, and atoms coming to assail them 

 equally in all directions ; but one side of the earth would be 

 partially screened by the sun, and the corresponding side of the 

 sun would be partially screened by the earth, so what we would 

 call the front faces of the earth and sun, which looked towards 

 one another, would be less bombarded by the atoms than all the 

 other faces. The atoms hitting at the back of the two bodies 

 would push them together. The atoms hitting the sides would 

 of course balance one another. The idea is ingenious, but re- 

 quires some strong assumptions. The attraction of gravitation 

 is not as the surface of the bodies, but as their mass. Lesage 

 had therefore to suppose his solid bodies not solid but excessively 

 porous, built up of molecules like cages, so that an infinite num- 

 ber of atoms went through and through them, allowing the last 

 layer of the sun or earth to be struck by just as many atoms as 

 the first, otherwise clearly the back part of the sun and earth 

 would gravitate more strongly than the front or nearer sides, 

 which would be struck only by the siftings of the previous 

 layers of matter. This notion involves a prodigious quantity of 

 material in the shape of flying atoms, where we perceive no 

 gross matter, but very little material in solid bodies where we 

 do find gross matter, and it further requires that the accumula- 

 tion of atoms which strike the solid bodies perpetually should 

 be insensible, and on these grounds, independently of dynamical 

 imperfections, we must reject the theory in its crude form, 

 though it may prove fruitful some day. Meanwhile it serves 

 to show that the school which denies action at a distance need 

 not have recourse to an absolute plenum. 



Three distinct atomic theories have now been discussed : we 

 have found believers in atoms of i solid singleness,' in atoms due 

 to the motion of a continuous fluid, and in atoms having the 

 property of exerting force at a distance. Naturally the three 

 elementary conceptions have been compounded in a variety 

 of ways. Leibnitz mentions with great disapproval a certain 

 Hartsoeker who supposed that atoms moved in an ambient fluid, 

 though the idea is not unlike his own. It is difficult to trace 

 the origin of the hypothesis, but Galileo and Hobbes both speak 

 of a subtle ether. The conception of an all-pervading imponder- 



