mrron; 





THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 

 AUGUST 1902. 



XVIII. On tlie Weights of Atoms*. 

 By Lord Kelvin, G.C. F.O.f 



§ 23. TTITHERTO in all our views we have seen nothing 

 XX of absolute dimensions in molecular structure, 

 and have been satisfied to consider the distance between 

 neighbouring molecules in gases, or liquids, or crystals, or 

 non-crystalline solids to be very small in comparison with 

 the shortest wave-length of light with which we have been 

 concerned. Even in respect to dispersion, that is to say, 

 difference of propagational velocity for different wave-lengths, 

 it has not been necessary for us to accept Cauchy's doctrine 

 that the spheres of molecular action are comparable with the 

 wave-length. We have seen that dispersion can be, and 

 probably in fact is, truly explained by the periods of our 

 waves of light being not infinitely great in comparison with 

 some of the periods of molecular vibration ; and, with this 

 view, the dimensions of molecular structure might, so far as 

 dispersion is concerned, be as small as we please to imagine 

 them, in comparison with wave-lengths of light. Never- 

 theless it is exceedingly interesting and important for in- 

 telligent study of molecular structures and the dynamics of 

 light, to have some well-founded understanding in respect to 

 probable distances between centres of neighbouring molecules 

 in all kinds of ponderable matter, while for the present at all 



* This is Lecture XVII. of my Baltimore Lectures, as now extended 

 and prepared for press. For convenience of reference the sectional numbers 

 have been retained as in the volume of Lectures. 



t Communicated bv the Author. 

 Phil. Mag. S. 6. Vol. 4. No. 20. Aug. 1902. N 



