CHAPTER XIX. 



THE EVIDENCE OF THE KINETIC THEORY AS TO 

 THE SIZE OF MOLECULES. 



412. WE may appropriately devote a final chapter to collecting and 

 discussing the evidence which has been obtained as to the size of the 

 molecular structure. 



Throughout the book we have examined to what extent observations are 

 in agreement with theoretical predictions, calculated on the supposition that 

 the molecules may be treated as hard elastic spheres. We have also had 

 another picture of a molecule before us namely, a point-centre of force, the 

 law of force between two molecules being that of the inverse fifth power 

 of the distance. Neither of these two representations were supposed to 

 reproduce all the properties of the molecule ; we found it necessary continually 

 to bear in mind that they were at best only approximations, and to examine 

 how the imperfections in our representation of a molecule would affect the 

 results arrived at with the help of this representation. 



413. The question which we must attempt to answer first is as to which 

 of these two representations gives the better picture of a molecule. In 

 deciding this question the following table will be of use : 



As regards each ratio it will be seen that the best approximations to the 

 observed values are given by the hypothesis of elastic spheres. 



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