THE STORY OF NINETEENTH-CENTURY SCIENCE 



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 bet- 

 ter than this ; unless, indeed, one other result that fol- 

 lowed 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 England, 

 independently. 



The work is purely mathematical, of course, but the 

 results are regarded as unassailable ; indeed, Lord Kelvin 

 speaks of them as being absolutely demonstrative within 

 certain limits of accuracy. This does not mean, how- 

 ever, that they show the exact dimensions of the mole- 

 cule ; 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 one 

 one-hundred-millionth of a centimetre for the minimum. 

 Such figures convey no particular meaning to our blunt 

 senses, but Lord Kelvin has given a tangible illustration 

 that aids the imagination to at least a vague comprehen- 

 sion of the unthinkable smallness of the molecule. He 

 estimates that if a ball, say of water or glass, about " as 

 large as a football, were 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 shot, but proba- 

 bly less coarse-grained than a heap of footballs." 



Several other methods have been employed to estimate 



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