188 SPECULATIVE SCIENCE 



his head at this explanation, and would have much preferred 

 the theory just started by Sir William Thomson, and long since 

 vaguely suggested by Hobbes, that the elasticity of atoms may 

 be due to the motion of their parts a proposition exemplified 

 by one smoke-ring bounding away from another in virtue of the 

 relative motions of their parts, these not being necessarily them- 

 selves elastic. The energy of the molecule at that point where 

 it strikes its neighbour and changes velocity is on this theory 

 transferred to another part of the molecule which moves faster 

 as the first part moves more slowly. If the molecules of gross 

 matter are made up of atoms in rapid motion, as Lucretius 

 believed, or of a portion of whirling fluid, as Sir William Thomson 

 suggests, and if elasticity itself be only a secondary property, 

 not possessed by the primordia rerum at all, then the proposition 

 that a molecule never can come to rest is undoubtedly true ; 

 such rest would be equivalent to the destruction of matter. 

 Lucretius could not have proved this, nor even have understood 

 the proof. He did not know the laws of motion even of two 

 elastic bodies, but it is singular to find modern science return- 

 ing to the never-ending motion of the old Greek atom. 



The next proposition of our author explains the varying den- 

 sity of bodies. He says that the greater or less density of bodies 

 depends on the smaller or greater distance to which the atoms 

 in each continue to rebound after striking one another. They 

 never stop striking and rebounding ; they are in perpetual 

 motion, tossed about by blows. Mr. Munro's translation fails, 

 it seems to us, to convey this view, reading as though the atoms 

 struck, rebounded and remained quiet afterwards, hooked as it 

 were together ; but Lucretius in many passages describes the 

 never-ending restlessness of his atoms, tossed like motes in a 

 sunbeam, which he describes to illustrate the motion of the 

 atoms in void. This explanation of the varying density of 

 matter is still commonly received, and will be found in all popu- 

 lar text-books ; the density of the ultimate particles of gravi- 

 tating matter is very generally assumed to be the same, the 

 greater or less density of gross matter being supposed due to 

 empty pores, of greater or smaller magnitude, separating the 

 molecules. At first sight it is very difficult to see how any 

 other explanation of varying density can be given, since we 



