222 
Proceedings of the Royal Society of Edinburgh. [Sess. 
Part II. — Case oe the Diatomic Molecule. 
Introduction. 
The method of finding the molecular refractivity of a molecule by 
the addition of the atomic refractivities of the atoms in the molecule 
gives very accurate results in the case of organic compounds containing 
carbon, hydrogen, oxygen, and halogen, provided care is taken to use the 
values of carbon and oxygen corresponding to their chemical bindings 
in the molecule. This method is used, therefore, as an aid to the deter- 
mination of the chemical constitution of organic compounds (vide chaps, 
viii and ix of S. Smiles’ book on the Relation of Chemical Consti- 
tution to some Physical Properties). Incidentally we may note that it 
is this conformity to the additive law in the case of organic compounds 
which justifies such use of experimental results for purposes of verification 
as has been made in Part I of this analysis. In the case of organic 
compounds containing nitrogen, and in that of inorganic salts, the law 
does not hold to the same degree of approximation. 
E.g. Value of molecular refractivity of — 
(1) hydrochloric acid = M(HCl) = 6’70 by observation, and 
= 6*83 when calculated from H-f Cl, 
giving a difference of + 2 per cent. 
(2) M(H 2 0) by observation = 3 ‘82, 
and M(H 2 0) calculated from H 2 + 0 = 4T4. 
Difference per cent. = + 8-4. 
(3) M(NO) by observation = 4*46, 
M(NO) calculated from N + O = 4 - 25, 
giving a difference of — 5 per cent. 
A good many cases of this nature are given by Briihl in the paper 
already cited, but the above cases will do for purposes of illustration of 
the value of the discrepancy in the additive law which is encountered in 
the case of gaseous compounds. 
In this part of the paper it is intended to discuss, in a very approximate 
manner, the divergence from the additive law which will arise when the 
contribution to the molecular refractivity of a diatomic molecule due to 
electrical action between the atoms in the molecule is taken into account. 
(a) We shall denote the values of the quantities pertaining to the 
two atoms of the diatomic molecule by the suffixes 1 and 2 respectively. 
Let atom I contain ^ — n^.s electrons, and atom 2 contain /j, 2 = n 2 -\- s 
