THE SIZE OF ATOMS. 193 



by Cauchy's unmodified theory, and by looking 

 back to the little table of calculated results 

 (Table II. p. 184), you will see that there could not 

 be more than 12 molecules in the wave-length 

 of violet light in water or alcohol ; say 10 in 

 hard crown glass ; 8 in flint glass ; and in 

 carbon disulphide actually not more than 4 

 molecules in the wave-length, if we are to depend 

 upon Cauchy's unmodified theory for the explana- 

 tion of dispersion. So large coarse-grainedness 

 of ordinary transparent bodies, solid or fluid, is 

 quite untenable. Before I conclude, I intend to 

 show you, from the kinetic theory of gases, a 

 superior limit to the size of molecules, according 

 to which, in glass or in water, there is probably 

 something like 600 molecules to the wave-length, 

 and almost certainly not fewer than 200, or 300, or 

 400. But even without any such definite estimate 

 of a superior limit to the size of molecules, there 

 are many reasons against the admission that it is 

 probable or possible there can be only four, or 

 five, or six, to the wave-length. The very drawing, 

 by Nobert, of 4000 lines on a breadth of a milli- 



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