448 Dr. R. D. Kleeman on 



approximately the same relation to one another as the 



relative values of — o-iven in the 8th column. It is also 



o-i & 

 seen that the axis at right angles to the plane in which the 

 atoms lie is, as we should expect, much smaller than an axis 

 in that plane. Thus it appears that the atoms of a molecule 

 lie approximately in one plane. 



We should expect, however, from this that the ratio for 

 ethyl propionate given in the 8th column should be greater 

 than that for any of the other substances given in the table. 

 But this is not the case. Thus the atoms o£ an ethyl- 

 propionate molecule do not lie altogether in a plane. I£ the 

 atoms are in rotation round the centre o£ the molecule, we 

 should expect that greater stability would be secured if, in a 

 molecule containing many atoms, the latter did not lie 

 exactly in a plane. 



We may therefore conclude that instead o£ supposing the 

 molecule a sphere, we shall be nearer the truth in supposing 

 it an oblate spheroid of which the ratio of the axis of the 

 circular section to that at right angles to this section is 



, (SmV3)3/2 

 g^nby-^^-. 



The values of this ratio have been calculated for a number 

 of compounds, and are contained in Table II. It is very 

 probable that it may be possible to connect them with other 

 properties of the substances such as crystalline form, and 

 therefore a table of their values seems useful. 



It is of interest to inquire whether the shape of the molecule 



changes with temperature. From the equation rj = 2 



we see that if the radius of the sphere of action, cr 1? is inde- 

 pendent of the temperature, rj is proportional to v, and 

 therefore to T 1 ' 2 , since v is proportional to T 1/2 . Experiment, 

 however, shows that rj is approximately proportional to the 

 first power of the temperature, so that a 1 must be a function 

 of the temperature. If there is no field of force surrounding 

 the molecule this would indicate that the volume of the 

 molecule changes with the temperature. If, however^we 

 regard the molecules as centres of force, their apparent 

 cross-section will change on account of the increase in the 

 velocity of the colliding molecules with temperature. Max- 

 well on this supposition proved that the force of attraction 

 varied inversely as the fifth power of the distance from the 

 centre of the molecule. We thus see that the variations of 



* Loc. cit. 



