LUCRETIUS AND THE ATOMIC THEORY 189 



find that by compression we actually can increase the density 

 of bodies without altering their weight or mass in any way. 

 Now, unless there were a void space separating the molecules, 

 where can these go to when squeezed ? Most men 1 will find a 

 difficulty in conceiving that space absolutely full of matter, soft 

 or hard, can be made to hold more ; but the same space does 

 hold sometimes more and sometimes less gross matter, so that in 

 the latter case it cannot be quite full, or, in other words, the body 

 it contains is composed in part of empty pores. The proof is 

 incomplete, and, if molecules be formed by the motion of a fluid, 

 greater density may possibly be due to a modification in the 

 motion of molecules, and not only to the greater frequency of 

 the eddying molecules in a given space. 



Lucretius next points out that his atoms must move very 

 rapidly. In vacuum atoms travel faster than light. His proof 

 of this is extremely vague. He says the light and the heat of 

 the sun (which he calls ' vapours ') are forced to travel slowly, 

 cleaving the waves of air, and several minute bodies of the heat 

 (vapour) are entangled together and impede one another, but 

 atoms of solid singleness can go ahead wholly unimpeded in a 

 vacuum not a very satisfactory proof. The idea running in 

 the mind of the writer seems to have been that any matter 

 moving in a medium would be impeded by friction, and there- 

 fore necessarily move more slowly than a free atom moving in 

 a void ; he may also have felt that, if all the power of the uni- 

 verse depended on the motion of exceedingly small particles, it 

 was necessary to suppose them endowed with great velocity; 

 but we do not find this argument used, although it has led the 

 modern believers in atoms to the conviction that if their motion 

 does represent energy, their velocity must be enormous. Lucre- 

 tius would be glad to know that Herapath, Joule, Kronig, 

 Clausius, and Clerk Maxwell have been able to calculate it; 

 4 * inch is the distance named by Maxwell. 



The nature of the original motion of atoms is next defined. 

 Atoms which have not struck one another move in straight 

 parallel lines, sheer downwards ; gravitation is the evidence of 

 this. An infinite number of atoms eternally pour from infinite 



1 Sir William Thomson and Professor Tait find no difficulty in this con- 

 ception. 



