278 THE POPULAR SCIENCE MONTHLY. 



Every substance, simple or compound, has its own molecule. It 

 this molecule be divided, its parts are molecules of a different sub- 

 stance or substances from that of which the whole is a molecule. An 

 atom, if there is such a thing, must be a molecule of an elementary 

 substance. Since, therefore, every molecule is not an atom, but every 

 atom is a molecule, I shall use the word molecule as the more general 

 term. 



I have no intention of taking up your time by expounding the 

 doctrines of modern chemistry with resj>ect to the molecules of differ- 

 ent substances. It is not the special but the universal interest of 

 molecular science which encourages me to address you. It is not 

 because we happen to be chemists or physicists or specialists of any 

 kind that we are attracted toward this centre of all material exist- 

 ence, but because we all belong to a race endowed with faculties 

 which urge us on to search deep and ever deeper into the nature of 

 things. 



We find that now, as in the days of the earliest physical specula- 

 tions, all physical researches appear to converge toward the same 

 point, and every inquirer, as he looks forward into the dim region 

 toward which the path of discovery is leading him, sees, each accord- 

 ing to his sight, the vision of the same quest. 



One may see the atom as a material point, invested and surrounded 

 by potential forces. Another sees no garment of force, but only the 

 bare and utter hardness of mere impenetrability. 



But though many a speculator, as he has seen the vision recede 

 before him into the innermost sanctuary of the inconceivably little, 

 has had to confess that the quest was not for him, and though phi- 

 losophers in every age have been exhorting each other to direct their 

 minds to some more useful and attainable aim, each generation, from 

 the earliest dawn of science to the present time, has contributed a due 

 proportion of its ablest intellects to the quest of the ultimate atom. 



Our business this evening is to describe some researches in molecu- 

 lar science, and in particular to place before you any definite informa- 

 tion which has been obtained respecting the molecules themselves. 

 The old atomic theory, as described by Lucretius and revived in 

 modern times, asserts that the molecules of all bodies are in motion, 

 even when the body itself appears to be at rest. These motions of 

 molecules are, in the case of solid bodies, confined within so narrow a 

 range that even with our best microscopes we cannot detect that they 

 alter their places at all. In liquids and gases, however, the molecules 

 are not confined within any definite limits, but work their way 

 through the whole mass, even when that mass is not disturbed by any 

 visible motion. 



This process of diffusion, as it is called, which goes on in gases and 

 liquids and even in some solids, can be subjected to experiment, and 

 forms one of the most convincing proofs of the motion of molecules. 



