462 THE POPULAR SCIENCE MONTHLY. 



THE MOLECULAR THEORY * 



By LE EOY C. COOLEY, Ph. D. 



THE idea that matter is an aggregate of minute particles, each of 

 which possesses all the essential properties of the mass, is as old 

 as Democritus, but it was left for the present century to crystallize the 

 conception of the atom in clear and accurate expression. The atomic 

 theory, revived and vitalized by the illustrious Dalton, has not simply 

 been able to survive the conflicts in which many an older theory has 

 been wrecked : it has itself been a prime mover of revolutions. It is 

 doubtful whether without it the recent advances in chemical and physi- 

 cal science could have been made. 



But the atom in chemistry is not the atom in physics ; they are of a 

 different order. When the idea of a chemical atom came to be clearly 

 conceived so that atom could be defined, as it is, to mean the smallest 

 particle of an elementary substance which can enter into the composi- 

 tion of a compound, the most natural, if not, indeed, the inevitable 

 corollary would be that the compound itself must be made up of parts, 

 each of which, containing only the minimum number of its constituent 

 atoms competent to give it character, must be the smallest particle of 

 that substance which can possibly exist. To distinguish this minutest 

 portion of a substance from the chemical atoms of which it is com- 

 posed, the French called it the molecule literally the little mass ; and 

 this word molecule, homeless in the English language less than one 

 hundred years ago, expresses an idea which now lies at the foundation 

 of modern physics. 



It has been said that the science of astronomy is the demonstration 

 of the law of gravitation. Indeed, what evidence have we of the 

 truth of Newton's grand generalization, except that it explains the 

 phenomena of the skies ? So, in the outset, we may say that the 

 science of physics is the demonstration of the molecular theory of the 

 constitution of matter, since it explains phenomena, suggests research, 

 directs experiment, classifies and unitizes wide ranges of apparently 

 diverse results, to an extent unparalleled by any other. 



This theory boldly affirms a limit to the divisibility of matter, and 

 thus seems to defy the logic of the metaphysician, who, passing the 

 limit set by the necessary imperfections of manipulation, carries the 

 process of subdivision mentally downward through the scale of little- 

 ness, until, finding no place where his conception of subdivision must 

 halt, declares that no limit exists. But the physicist does not deny the 

 logic of the metaphysician ; he simply remembers that mental concep- 

 tions need not of necessity represent the realities of nature's pi-ocesses, 



* Read before the Poughkeepsie Society of Natural Science, December, 1878. 



