454 Prof. J. C. Maxwell on Molecules. 



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 we have no doubt that the subdivision might 

 be carried further if our senses were more acute and our instru- 

 ments more delicate. Thus far all are agreed; but now the 

 question arises, Can this subdivision be repeated for ever ? 



According to Democritus and the atomic school, we must 

 answer in the negative. After a certain number of subdivisions 

 the drop would be divided into a number of parts each of which 

 is incapable of further subdivision. We should thus in imagi- 

 nation 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 divi- 

 sible, so are its parts down to the minutest subdivisions, 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 organized bodies such as men 

 and animals ; but he maintained that those inorganic substances 

 which appear to us homogeneous are really so, and that the uni- 

 versal experience of mankind testifies that every material body 

 without exception is divisible. 



The doctrine of atoms and that of homogeneity are thus in 

 direct contradiction. 



But we must now go on to molecules. Molecule is a modern 

 word. It does not occur in Johnson's Dictionary. The id^o.s 

 it embodies are those belonging to modern chemistry. 



A drop of water (to return to our former example) may be 

 divided into a certain number, and no more, of portions similar 

 to each other. Each of these the modern chemist calls a mole- 

 cule of water. But it is by no means an atom, for it contains 

 two different substances, oxygen and hydrogen ; and by a cer- 

 tain process the molecule may be actually divided into two parts, 

 one consisting of oxygen and the other of hydrogen. According 

 to the received doctrine, in each molecule of water there are two 

 molecules of hydrogen and one of oxygen. Whether these are 

 or are not ultimate atoms I shall not attempt to decide. 



We now see what a molecule is, as distinguished from an atom. 



A molecule of a substance is a small bodv such that if, on the 



