THE PROGRESS OF PHYSICS. 173 



D. Argument from the Behaviour of Gases. 

 Clausius and Maxwell deduced theoretically the con- 

 clusion that the length of the mean free path of a 

 moving particle in a gas (i.e., the distance which it 

 will pass through between every two successive colli- 

 sions), divided by the diameter of any one particle, is 

 equal to the ratio of the whole space occupied by the 

 particles to about eight and a half times the bulk of 

 the whole particles.* In various ways it was found 

 possible to form an equation with approximate data, 

 and the result comes out that the diameter of a par- 

 ticle is not very different from ^T^Tnnr.innr of an inch. 



As a good-sized plum or a small orange is to the 

 whole earth, so is the coarse-grained particle to a 

 drop of water of an inch in diameter. 



The calculations of Joule and Clausius, Maxwell 

 and Boltzmann lead to such statements as the follow- 

 ing: " Atoms are big things, the thousand millionth 

 of an inch in diameter, and they cannot travel far 

 without mutual collisions. They are constantly col- 

 liding, even in a very good vacuum. In ordinary 

 air every atom strikes another about six thousand 

 million times a second, and it cannot travel even a 

 microscopic distance without collision; its free path 

 is microscopic, or on the average ultra-microscopic." f 



E. From Electrical Phenomena. As Prof. Oliver 

 Lodge says, " atoms are big things " " the thou- 

 sand millionth of an inch in diameter, and they can- 

 not travel far without mutual collisions." Much too 

 big and cumbrous these are to figure in an interpre- 

 tation of the cathode rays, the Lenard rays, the 

 Eontgen rays! For here we are brought face to 



* See Recent Advances, p. 316. 



t Oliver Lodge. Modern Views of Matter. International 

 Monthly, I. (1900), p. 515. 



