Prof. J. C. Maxwell on Molecules. 457 



This is the complete dynamical explanation of the fact disco- 

 vered 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 in- 

 creased. Each molecule will now strike the sides of the vessel 

 a greater number 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 pressure due to each molecule will vary 

 as the square of the velocity. Now the increase of velocity cor- 

 responds, on our theory, to a rise of temperature; and in this 

 way we can explain the effect of warming the gas, and also the 

 law discovered by Charles, that the proportional expansion of all 

 gases between given temperatures is the same. 



The dynamical theory also tells us what will happen if mole- 

 cules of different masses are allowed to knock about together. 

 The greater masses will go slower than the smaller ones, so that 

 on an average every molecule, great or small, will have the same 

 energy of motion. 



The proof of this dynamical theorem, in whieh I claim the 

 priority, has recently been greatly developed and improved by 

 Dr. Ludwig Boltzmann. The most important consequence 

 which flows from it is that a cubic centimetre of every gas at 

 standard temperature and pressure contains the same number of 

 molecules. This is the dynamical explanation of Gay-Lussac's 

 law of the equivalent volumes of gases. But we must now de- 

 scend to particulars, and calculate the actual velocity of a mole- 

 cule of hydrogen. 



A cubic centimetre of hydrogen, at the temperature of melting * 

 ice and at a pressure of one atmosphere, weighs 000008954 

 gramme. We have to find at what rate this small mass must 

 move (whether altogether or in separate molecules makes no dif- 

 ference) so as to produce the observed pressure on the sides of 

 the cubic centimetre. This is the calculation which was first 

 made by Dr. Joule ; and the result is 1859 metres per second. 

 This is what we are accustomed to call a great velocity. It is 

 greater than any velocity obtained in artillery practice. The 

 velocity of other gases is less, as you will see by the Table ; but 

 in all cases it is very great as compared with that of bullets. 



We have now to conceive the molecules of the air in this hall 

 flying about in all directions at a rate of about seventy miles in 

 a minute. 



If all these molecules were' flying in the same direction they 

 would constitute a wind blowing at the rate of seventy miles 



