636 Mr. W. Sutherland on the 



to unit mass it is 1/ra where m is the actual mass of a molecule, 

 and so N = 1/MA where h is the actual mass of an atom 

 of hydrogen, the tabulated values of M 2 / are really values of 

 *? 2 a ,2 //a 2 . But ejh is a standard electrolytic constant. Hence 

 it follows that in investigating the laws of (M 2 /)*, as in some 

 of the communications referred to, we were really studying 

 the law of s the distance between # and b in the doublets 

 which form the chemical bonds. The electric theory of 

 molecular attraction leads thus to the simplest possible 

 physical interpretation of the parameters of molecular force, 

 and invests their laws with a more immediate interest. 



The best test to apply to the theory at this stage is to 

 calculate the order of magnitude of s to see whether it is 

 consistent with what we know of molecular sizes. The linear 

 dimensions of molecules are of the order 10 -8 cm., and of 

 electrons (Phil. Mag. [5 J xlvii.) of the order 10 -14 , and these 

 are limits for the size of s. Let us take the simplest type of 

 binary molecule such as NaCl, for which the tabulated value 

 of (M 2 f)* (xxxix.) is 5*6, which is to be multiplied by 10 6 to 

 give the value when the dyne is the unit of force. Now 



es/h=(my and A/e = 345 x 10~ 17 , 



.-. for NaCl 5 = r93xl0- 8 . 



To determine the linear dimensions of the NaCl molecule we 

 can proceed as for that of the Li atom in " Ionization &c. " 

 (Phil. Mag. [6] iii. p. 176) where the radius of the Li atom 

 is found from its ionic velocity in water to be 2 x 10~ 9 . The 

 volume of the Na atom is 7*4/2 times that of Li, and of CI is 

 19/2 times (see Table III. of that paper), so that the mean 

 radius of NaCl will be (26-4/2)* x 2 x 10- 9 =7'26 x 10~ 9 cm. 

 The diameter d of the NaCl molecule is thus found to be 

 1*45 X 10 -8 . The fact that we have found 5 a little larger than 

 d indicates that we have overshot the mark in reducing 

 molecular attraction so that it operates between only imme- 

 diate neighbours at their average distance apart. But from 

 the nature of the case we can expect to obtain only the order 

 of magnitude of s, which is about equal to that of molecular 

 diameters. 



3. Relation to Helmholtzs Electric Theory of 

 Chemical Valence. 



It is of great importance in chemical dynamics that we 

 should be able to find accurately the ratio of s to d in order 

 to push farther with Helmholtz's theory that the chemical 

 forces between atoms are identical with the forces between 

 the electric charges constituting their valencies. Richarz 



