1883.] on the Size of Atoms. 213 



an actual diameter l-10,000,000th of a centimetre, represents a gas in 

 which a condensation of 1 to 10 linear, or 1 to 1000 in bulk, would 

 bring the molecules close tog;ether. 



Now you are to imagine the particles moving in all dii'ections, 

 each in a straight line until it collides with another. The average 

 length of free path is 10 centimetres in our diagram, representing 

 l-100,000th of a centimetre in reality. And to suit the case of atmo- 

 spheric 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 

 l-5,000,000,000th 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,* and I, unconsciously following in his wake, 

 have come to the same conclusion ; that is, we have arrived at the 

 absolute certainty that the dimensions of a molecule of air are something 

 like that which I have stated. 



The four lines of argument which I have now indicated lead all to 

 substantially the same estimate of the dimensions of molecular 

 structure. Jointly they establish, with what we cannot but regard as 

 a very high degree of probability, the conclusion that, in any 

 ordinary liquid, transparent solid, or seemingly ojiaque solid, the 

 mean distance between the centres of contiguous molecules is less than 

 the l-5,000,000th, and greater than the l-l,000,000,000th of a centi- 

 metre. 



To form some conception of the degree of coarse-grainedness 

 indicated by this conclusion, imagine a globe of water or glass, as 

 large as a football,t to be magnified up to the size of the earth, each 

 constituent molecule being magnified in the same proportion. The 

 magnified structure would be more coarse-grained than a heap of 

 small shot, but probably less coarse-grained than a heap of footballs. 



[W. T.| 



♦ Sitzungshericlde of the Vienna Academy, Oct. 12, 1865, p. 395, 

 t Or say a globe of 16 centimetrea diaiueter. 



