ATOM. 473 



If this is the true opinion of Lucretius, and if the downward flight of the 

 atoms arises, in his view, from their own gravity, it seems very doubtful whether 

 he attributed the weight of sensible bodies to the impact of the atoms. The 

 latter opinion is that of Le Sage, of Geneva, propounded in his Lucrece New- 

 tonien, and in his Traite de Physique Mecanique, published, along with a second 

 treatise of his own, by Pierre Prevost, of Geneva, in 1818*. The theory of 

 Le Sage is that the gravitation of bodies towards each other is caused by the 

 impact of streams of atoms flying in all directions through space. These atoms 

 he calls ultramundane corpuscules, because he conceives them to come in all 

 directions from regions far beyond that part of the system of the world which 

 is in any way known to us. He supposes each of them to be so small that 

 a collision with another ultramundane corpuscule is an event of very rare occur- 

 rence. It is by striking against the molecules of gross matter that they dis- 

 charge their function of drawing bodies towards each other. A body placed by 

 itself in free space and exposed to the impacts of these corpuscules would be 

 bandied about by them in all directions, but because, on the whole, it receives 

 as many blows on one side as on another, it cannot thereby acquire any sensible 

 velocity. But if there are two bodies in space, each of them will screen the 

 other from a certain proportion of the corpuscular bombardment, so that a 

 smaller number of corpuscules will strike either body on that side which is next 

 the other body, while the number of corpuscules which strike it in other direc- 

 tions remains the same. 



Each body will therefore be urged towards the other by the effect of the 

 excess of the impacts it receives on the side furthest from the other. If we 

 take account of the impacts of those corpuscules only which come directly from 

 infinite space, and leave out of consideration those which have already struck 

 mundane bodies, it is easy to calculate the result on the two bodies, supposing 

 their dimensions small compared with the distance between them. 



The force of attraction would vary directly as the product of the areas of 

 the sections of the bodies taken normal to the distance and inversely as the 

 square of the distance between them. 



Now, the attraction of gravitation varies as the product of the masses of 

 the bodies between which it acts, and inversely as the square of the distance 

 between them. If, then, we can imagine a constitution of bodies such that 



* See also Constitution de la Matiere, &c., par le P. Leray, Paris, 1869. 



VOL. II. 60 



