CH. xxxv. MOLECULAR THEORY OP GASES. 363 



the seventeenth century, was the first to suggest that gases 

 are formed of free particles or molecules in constant motion, 

 and that the pressure of a gas upon the sides of the vessel 

 containing it, is caused by the agitation of molecules within. 

 Thus, for example, an ordinary air-ball, if held near the fire, 

 expands, because the molecules of the air within move with 

 more energy and beat with more force against the sides of 

 the bag containing them. 



In our time Graham, Joule and Andrews, Clausius, 

 Clerk -Maxwell and Boltzmann, Helmholtz and Sir W. 

 Thomson, have enabled us to see these molecules with our 

 imagination, dashing to and fro, hitting one against the other, 

 and flying off in different directions and at different rates. 

 They have ascertained, for example, that in hydrogen gas, at 

 the ordinary temperature and pressure of our atmosphere, 

 each molecule of hydrogen is moving at the rate of about 

 6225 feet in a second, yet it must perform all its motions 

 in an incredibly small space, for it is constantly striking 

 against some other molecule, so that in that second of time it 

 rebounds more than seventeen thousand million times. This 

 number of collisions of the molecules will always correspond 

 to the same temperature and pressure of the gas. But if 

 more energy be imparted to the molecules the temperature 

 will rise, for the more quickly they dash to and fro and 

 strike each other the higher the temperature will be. When 

 we heat a gas we are in fact increasing the energy of the 

 molecules and making them dash to and fro more rapidly. 

 This rebounding of the molecules is the reason, as Clausius 

 showed, why, when two gases are brought together, it takes 

 them some time to mix thoroughly, the new molecules have 

 to fight their way among those which are already in the 

 field. Dr. Graham had long before shown experimentally 

 that the rate of diffusion is inversely as the square roots of 



