[From Nature, Vol. Till.] 



LXII. Molecules*. 



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 subjected 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 organisation 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 realised, 

 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 



* A Lecture delivered before the British Association at Bradford. 

 VOL. II. 46 



