364 MOLECULES. 



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 

 tliat 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 intellects 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 definite information 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. 



Now the recent progress of molecular science began with the study of the 

 mechanical effect of the impact of these moving molecules when they strike 

 against any solid body. Of course these flying molecules must beat against what- 

 ever is placed among them, and the constant succession of these strokes is, according 

 to our theory, the sole cause of what is called the pressure of air and other gases. 



This appears to have been first suspected by Daniel Bernoulli, but he had not 

 the means which we now have of verifying the theory. The same theory was 

 afterwards brought forward independently by Lesage, of Geneva, who, however, 

 devoted most of his labour to the explanation of gravitation by the impact of 

 atoms. Then Herapath, in his Mathematical Physics, published in 1847, made a 

 much more extensive application of the theory to gases, and Dr Joule, whose 

 absence from our meeting we must all regret, calculated the actual velocity of 

 the molecules of hydrogen. 



