i8o SPECULATIVE SCIENCE 



components. This statement is complementary to the first. To- 

 gether the two propositions affirm that constancy in the total 

 quantity of matter which is a commonplace truth now, but 

 which to Lucretius must have been unsupported by any rigorous 

 proof. His own arguments in support of the law go no farther 

 than to show that we have no proof of the destruction of any 

 portion of matter. He shows that rain when it falls is not lost, 

 but produces leaves and trees, that ' by them in turn our race 

 and the race of wild beasts is fed ; ' but he makes no effort to 

 measure accurately the quantity of matter apparently disappear- 

 ing, but reappearing in the new form, and without that measure- 

 ment his proposition could not be rigorously proved ; moreover, 

 in the mind of Lucretius, the indestructibility referred to all 

 kinds of causes ; so that, to make our proposition co-extensive 

 with his, we must interpret it to mean that matter is indestruc- 

 tible, and that no cause fails to produce an equivalent effect, 

 though Lucretius probably did not conceive these two parts of 

 his proposition separate one from the other. 



Occasion is taken at this point to state that the components 

 into which bodies are resolved, or out of which they are built, 

 may be invisible. The third distinct proposition states that 'all 

 things are not on all sides jammed together and kept in by 

 body : there is also void in things.' Lucretius thought that, in 

 order to explain the properties of matter, it was absolutely 

 necessary to admit the existence of vacuum, or empty space, 

 containing nothing whatever. If there were not void, he says, 

 things could not move at all ! And it does seem, at first sight, 

 that in a universe absolutely full, like a barrel full of herrings, 

 so shaped as to leave not a cranny between them, no motion 

 whatever would be possible ; but reflection shows us that what 

 is called re-entering motion is possible, even under those cir- 

 cumstances, provided we do not suppose our fish to stick to one 

 another ; there may be an eddy in which the fish swim round 

 and round one after the other, without leaving any vacant space 

 between them or on either side, and yet without enlarging, 

 diminishing, or disturbing the barrel as they move. 1 Lucretius 

 either failed to perceive this, or declined to admit the possi- 

 bility that all the movements of gross matter could be of this 

 1 A homogeneous plenum may also be conceived as compressible. 



