Prof. J. C. Maxwell on Molecules. 455 



one hand, a number of similar molecules were assembled together, 

 they would form a mass of that substance ; while, on the other 

 hand, if any portion of this molecule were removed, it would no 

 longer be able, along with an assemblage of other molecules 

 similarly treated, to make up a mass of the original substance. 



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

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

 substance or substances from that of which the whole is a mole- 

 cule. 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 mo- 

 lecule as the more general term. 



I have no intention of taking up your time by expounding 

 the doctrines of modern chemistry with respect to the molecules 

 of different 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 towards this 

 centre of all material existence, 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 spe- 

 culations, all physical researches appear to converge towards the 

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

 region towards which the path of discovery is leading him, sees, 

 each according to his sight, the vision of the same quest. 



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

 rounded 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 incon- 

 ceivably little, has had to confess that the quest was not for him ; 

 and though philosophers 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 in- 

 tellects to the quest of the ultimate atom. 



Our business this evening is to describe some researches in 

 molecular science, and in particular to place before you any de- 

 finite information which has been obtained respecting the mole- 

 cules themselves. The old atomic theory, as described by Lu- 

 cretius 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 



