A HISTORY OF SCIENCE 



urements were confirmed by study of the rate of diffu- 

 sion at which different gases mix together, and also by 

 the rate of diffusion of heat through a gas, both these 

 phenomena being chiefly due to the helter-skelter flight 

 of the molecules. 



It is sufficiently astonishing to be told that such 

 measurements as these have been made at all, but the 

 astonishment grows when one hears the results. It ap- 

 pears from Clerk-Maxwell's calculations that the mean 

 free path, or distance traversed by the molecules between 

 collisions in ordinary air, is about one-half-millionth of 

 an inch; while the speed of the molecules is such that 

 each one experiences about eight billions of collisions 

 per second ! It would be hard, perhaps, to cite an illus- 

 tration showing the refinements of modern physics 

 better than this; unless, indeed, one other result that 

 followed directly from these calculations be considered 

 such the feat, namely, of measuring the size of the 

 molecules themselves. Clausius was the first to point 

 out how this might be done from a knowledge of the 

 length of free path ; and the calculations were made by 

 Loschmidt in Germany and by Lord Kelvin in Eng- 

 land, independently. 



The work is purely mathematical, of course, but the 

 results are regarded as unassailable; indeed, Lord Kel- 

 vin speaks of them as being absolutely demonstrative 

 within certain limits of accuracy. This does not mean, 

 however, that they show the exact dimensions of the 

 molecule; it means an estimate of the limits of size 

 within which the actual size of the molecule may lie. 

 These limits, Lord Kelvin estimates, are about the one- 

 ten-millionth of a centimetre for the maximum, and the 



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