340 



Size of Molecules 



[CH. XIX 



This representation of a molecule has the further advantage that the 

 conceptions of " the diameter of a molecule," " a collision," " a free path," and 

 so on, have definite meanings when associated with hard spherical molecules, 

 but none at all when associated with point-centres of force. 



The Size of Molecules, regarded as Elastic Spheres. 



414. We accordingly agree for the present to regard molecules as elastic 

 spheres. Two series of calculations of the radii of these spheres have already 

 been given, the first (p. 141) from observed deviations from Boyle's Law, the 

 second (p. 251) from the observed coefficients of viscosity. To these can of 

 course be added calculations from the observed coefficients of conduction 

 of heat and diffusion. Collected together, the four sets of values are as 

 follows : 



VALUES OF io- x 10 8 . 



415. A further way of forming an estimate of the molecular radius is by 

 considering the density of the substance in the solid state. This way, how- 

 ever, only enables us to calculate an upper limit to the molecular radius. 



* The values for mercury vapour refer to 203 C., this being the lowest temperature at which 

 K has been determined. I have used Koch's value for K at 203 (K = -000395, Wied. Ann. xix. 

 p. 857) and Schleiermacher's value for ^, also at 203 (^='0000185, Wied. Ann. xxxvi. p. 346). 

 Meyer, from a consideration of the experimental difficulties, doubts the accuracy of the value of K 

 (Kinetic Theory, pp. 197, 296), and this doubt is confirmed by the facts that the ratio ^JKC V 

 is found to be as great as 3-15, and that the calculated values of fa are in disagreement with the 

 upper limit calculated in 415. 



