MOLECULES. 



the atomic theory, which, after its statement by Democritus, remained to be 

 done. Anaxagoras, in fact, stated a theory which so exactly contradicts the 

 atomic theory of Democritus that the truth or falsehood of the one theory 

 implies the falsehood or truth of the other. The question of the existence or 

 non-existence of atoms cannot be presented to us this evening with gr. 

 clearness than in the alternative theories of these two philosophers. 



Take any portion of matter, say a drop of water, and observe its properties. 

 Like every other portion of matter we have ever seen, it is divisible. Divide 

 it in two, each portion appears to retain all the properties of the original drop, 

 and among others that of being divisible. The parts are similar to the whole 

 in every respect except in absolute size. 



Now go on repeating the process of division till the separate portions of 

 water are so small that we can no longer perceive or handle them. Still \ve 

 have no doubt that the sub-division might be carried further, if our senses 

 were more acute and our instruments more delicate. Thus far all are a^: 

 but now the question arises, Can this sub-division be repeated for ever ? 



According to Democritus and the atomic school, we must answer in the 

 negative. After a certain number of sub-divisions, the drop would be di\ 

 into a number of parts each of which is incapable of further sub-division. We 

 should thus, in imagination, arrive at the atom, which, as its name literally 

 signifies, cannot be cut in two. This is the atomic doctrine of Democritus, 

 Epicurus, and Lucretius, and, I may add, of your lecturer. 



. According to Anaxagoras, on the other hand, the parts into which the drop 

 is divided are in all respects similar to the whole drop, the mere size of a 

 body counting for nothing as regards the nature of its substance. Hence if 

 the whole drop is divisible, so are its parts down to the minutest sub-divisions, 

 and that without end. 



The essence of the doctrine of Anaxagoras is that parts of a body are 

 in all respects similar to the whole. It was, therefore, called the doctrine 

 of Homoiomereia. Anaxagoras did not of course assert this of the parts of 

 organised bodies such as men and animals, but he maintained that those inor- 

 ganic substances which appear to u's homogeneous are really so, and that the 

 universal experience of mankind testifies that every material body, without 

 exception, is divisible. 



The doctrine of atoms and that of homogeneity are thus in direct con- 

 tradiction. 



