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LX. A Discourse on Molecules. By J. Clerk Maxwell, M.A., 

 F.R.S., Professor of Experimental Physics in the University 

 of Cambridge*. 



\ N atom is a body which cannot be cut in two. A molecule 

 -^- is the smallest possible portion of a particular substance. 

 No one has ever seen or handled a single molecule. Molecular 

 science, therefore, is one of those branches of study which deal 

 with things invisible and imperceptible by our senses, and which 

 cannot be subjected to direct experiment. 



The mind of man has perplexed itself with many hard ques- 

 tions. Is space infinite, and if so, in what sense ? Is the ma- 

 terial world infinite in extent, and are all places within that 

 extent equally full of matter ? Do atoms exist, or is matter in- 

 finitely divisible ? 



The discussion of questions of this kind has been going on 

 ever since men began to reason ; and to each of us, as soon as we 

 obtain the use of our faculties, the same old questions arise as 

 fresh as ever. They form as essential a part of the science of 

 the nineteenth century of our era as of that of the fifth century 

 before it. 



We do not know much about the science organization of Thrace 

 twenty-two centuries ago, or of the machinery then employed 

 for diffusing an interest in physical research. There were men, 

 however, in those days who devoted their lives to the pursuit of 

 knowledge with an ardour worthy of the most distinguished 

 members of the British Association; and the lectures in which 

 Democritus explained the atomic theory to his fellow-citizens of 

 Abdera realized, not in golden opinions only, but in golden 

 talents, a sum hardly equalled even in America. 



To another very eminent philosopher, Anaxagoras, best 

 known to the world as the teacher of Socrates, we are indebted 

 for the most important service to 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 greater clearness thau 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 



* Read before the British Association at Bradford, September 22, 1 H/ rq « 



