808 Forces of Attraction hetween Atoms and Molecules. 



molecule cannot therefore be smaller than the value of x given 

 by the equation 



which is obtained by equating the kinetic and potential 

 energies. In the case of two hydrogen molecules at 0° C. 

 this equation becomes 



2y7-1 y If)- 25 9 X 1()~ 46 



^lAJ^L^ x (1-84 x 10 5 ) 2 = , * 2 2 

 2 4# 4 



which gives # = 1'34 x 10~ 8 pm, This is of the same order as 

 the usually accepted diameter of a hydrogen molecule. 



Thus when two atoms on collision do not separate but 

 combine to form a molecule they are separated from one 

 another by a distance less than that of x given by the above 

 equation. 



The distance x with which two molecules of the same kind 

 would be separated when revolving round their centre of 

 gravity with their velocity of translation, is given by the 

 equation 



2i' 2 m K. v — 2 



— = ? (V-i) 2 . 



This equation is the same as the foregoing. When the 

 molecules are different they cannot both possess their original 

 velocity of translation. 



Therefore, when two atoms are combined into a molecule 

 they may rotate round their centre of gravity with a velocity 

 comparable with that of their translation without the equi- 

 librium being disturbed, 



Distances of Separation of the Atoms in a Molecule. 



In the investigation given in this paper the molecules in a 

 liquid were regarded as points, so that their diameter would 

 be small in comparison with their distance of separalion. 

 The effect produced by a molecule is then the sum of the 

 effects produced by its atoms, if these are approximaiely 

 independent of one another in their external action. Now 

 we have seen that the constant 2fy of the atoms of a molecule 

 is additive within less than 10 per cent. The diameter of a 

 molecule must therefore be less than one-tenth of the dis- 

 tance of separation of the molecules in a liquid at ordinary 

 temperatures. In the case of ether at f T c the distance of 

 separation of the molecules is 5'1 X 10~ 8 cm.., and the 



