276 THE POPULAR SCIENCE MONTHLY. 



THE THEORY OF MOLECULES. 1 



By Professor CLERK MAXWELL, F. R. S. 



AN 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 sub- 

 jected to direct experiment. 



The mind of man has perplexed itself with many hard questions. 

 Is space infinite, and if so in what sense ? Is the material world 

 infinite in extent, and are all places within that extent equally full of 

 matter ? Do atoms exist, or is matter infinitely 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 ai'dor 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 equaled 

 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 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. 



1 Lecture delivered before the British Association at Bradford. 



