456 Prof. J. C. Maxwell on Molecules. 



all. In liquids and gases, however, the molecules are not con- 

 fined 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 expe- 

 riment, and forms one of the most convincing proofs of the mo- 

 tion 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 Hying molecules must beat against whatever is placed 

 among them ; and the constant succession of these strokes is, ac- 

 cording to our theory, the sole cause of what is called the pres- 

 sure 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 in- 

 dependently 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/ pub- 

 lished in 1847, made a much more extensive application of the 

 theory to gases ; and Dr. Joule, whose absence from our Meet- 

 ing we must all regret, calculated the actual velocity of the mo- 

 lecules of hydrogen. 



The further development of the theory is generally supposed 

 to have begun with a paper by Kronig, which does not, however, 

 so far as I can see, contain any improvement on what had gone 

 before. It seems, however, to have drawn the attention of Pro- 

 fessor Clausius to the subject ; and to him we owe a very large 

 part of what has been since accomplished. 



We all know that air or any other gas placed in a vessel 

 presses against the sides of the vessel, and against the surface 

 of any body placed within it. On the kinetic theory this pres- 

 sure is entirely due to the molecules striking against these sur- 

 faces, and thereby communicating to them a series of impulses 

 which follow each other in such rapid succession that they pro- 

 duce an effect which cannot be distinguished from that of a con- 

 tinuous pressure. 



If the velocity of the molecules is given and the number 

 varied, then siuce each molecule on an average strikes the sides 

 of the vessel the same number of times, and with an impulse of 

 the same magnitude, each will contribute an equal share to the 

 whole pressure. The pressure in a vessel of given size is there- 

 fore proportional to the number of molecules in it— that is, to 

 the quantity of gas in it. 



