THE THEORY OF MOLECULES. 279 



Now, the recent progress of molecular science began with the 

 study of the mechanical effect of the impact of these moving mole- 

 cules when they strike against any solid body. Of course these flying 

 molecules must beat against whatever 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 afterward brought forward independently by 

 Lesage, of Geneva, who, however, devoted most of his labor 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. 



The further development of the theory is generally supposed to 

 have been 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 Prof. 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 pressure is entirely due 

 to the molecules striking against these surfaces, and thereby commu- 

 nicating to them a series of impulses which follow each other in such 

 rapid succession that they produce an effect which cannot be distin- 

 guished from that of a continuous pressure. 



If the velocity of the molecules is given, and the number varied, 

 thence since each molecule, on an average, strikes the side 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 therefore proportional to 

 the number of molecules in it, that is, to the quantity of gas in it. 



This is the complete dynamical explanation of the fact discovered 

 by Robert Boyle, that the pressure of air is proportional to its 

 density. It shows also that, of different portions of gas forced into a 

 vessel, each produces its own part of the pressure independently of 

 the rest, and this whether these portions be of the same gas or not. 



Let us next suppose that the velocity of the molecules is increased. 

 Each molecule will now strike the sides of the vessel a greater num- 

 ber of times in a second, but besides this, the impulse of each blow 

 will be increased in the same proportion, so that the part of the press- 

 ure due to each molecule will vary as the square of the velocity. 

 Now, the increase of the square of velocity corresponds, in our theory, 

 to a rise of temperature, and in this way we can explain the effect of 



