THE PROGRESS OF PHYSICS. 149 



be transformed into the energy of measurable motion 

 of measurably large or molar masses, heat may it- 

 self be " the energy of the directly immeasurable 

 movements of molecular (immeasurably small) 

 masses." 



Developments. "By applying calculations simi- 

 lar to those of Joule, but considerably extended by 

 the use of more powerful mathematical methods, such 

 as the methods of the theory of probabilities, Clausius 

 first, and, a little later, but far more profoundly, 

 Clerk Maxwell, and still more recently Boltzmann, 

 have arrived at very valuable results as to the motions 

 of swarms of impinging particles. One of the results 

 arrived at is that in a mass of hydrogen at ordinary 

 temperature and pressure, every particle has on an 

 average 17,700,000,000 collisions per second with 

 other particles ; that is to say, 17,700,000,000 times 

 in every second it has its course wholly changed. 

 And yet the particles are moving at a rate of some- 

 thing like 70 miles per minute. So comes this 

 curious problem given that the direction of motion 

 of a particle is arbitrarily changed 17,700,000,000 

 times in every second, and that the particle itself is 

 moving 70 miles in a minute, where would it be at 

 the end of a single minute, having started from any 

 given place? . . . The solution obtained is capable 

 of explaining almost everything that we know with 

 reference to the behaviour of gases, and perhaps 

 even of vapours." * 



SUMMAEY. The kinetic theory of gases, the 

 brilliant generalisation which harmonised the nu- 

 merous facts specific heat, diffusion, friction , 

 etc., known in regard to the behaviour of bodies in 

 a gaseous state, may be regarded as a corollary of the 

 * Tail's Recent Advances, 1876, pp. 324-5. 



