216 POPULAR LECTURES AND ADDRESSES. 



reality. And to suit the case of atmospheric air 

 of ordinary density and at ordinary pressure you 

 must suppose the actual velocity of each particle 

 to be 50,000 centimetres per second, which will 

 make the average time from collision to collision 

 i/5,ooo,ooo,oooth of a second. 



The time is so far advanced that I cannot 

 speak of the details of this exquisite kinetic 

 theory, but I will just say that three points 

 investigated by Maxwell and Clausius, viz. the 

 viscosity or want of perfect fluidity of gases, 

 the diffusion of gases into one another, and 

 the diffusion of heat through gases all these 

 put together give an estimate for the average 

 length of the free path of a molecule. Then 

 a beautiful theory of Clausius enables us, 

 from the average length of the free path, 

 to calculate the magnitude of the atom. That 

 is what Loschmidt has done, 1 and I, un- 

 consciously following in his wake, have come 

 to the same conclusion ; that is, we have 

 arrived at the absolute certainty that the 



1 Sitzungsberichte of the Vienna Academy, Oct. 12, 1865, p. 395. 



