466 Mr. S. S. Richardson on Magnetic Rotary 



obtains an exceedingly small fraction for the ratio of the 

 effective to the total number of molecules. It appears that 

 the frequency brought into play in these cases is due to 

 some accidental or seldom-recurring condition of the 

 .molecule. On account of their small number, electrons of 

 this type have little influence on the refractive index or 

 magnetic rotation, except quite close to the absorption band 

 to which they give rise, but are probably capable of giving 

 rise to fluorescence and phosphorescence. 



Class II. Valency electrons of dispersional type : — 



(a) Under this heading are included the electrons through 

 whose agency the attraction between the atoms in any given 

 molecule is produced. Eveiy such particle represents one 

 valency bond. 



(b) The contravalency electrons of certain elements, par- 

 ticularly N, P, S, 01, Br, I, may have a frequency sufficiently 

 low to affect the dispersion as well as the refraction. This 

 is more marked in the case of the elements which readily 

 part with these electrons, e. g. sulphur and iodine. 



Class III. Valency electrons of non-dispersional type. — 

 These differ from class II. in possessing a frequency so high 

 that their effect on the dispersion cannot be detected experi- 

 mentally, whilst they still exert a measurable influence on 

 the refraction. 



Class IV. Intra-atomic electrons. — This term may be used 

 for referring to electrons present in the atom which take no 

 part in deciding the valency. Such electrons will undergo 

 so little displacement relative to their positive complement, 

 that it is probable that each will exert only a very small 

 -effect on the refraction. 



An examination of the numerical values given in the 

 preceding tables shows the following relations hold good : — 



(a) The minimum number of electrons demanded by the 

 refraction calculated on the normal value of e/m is never 

 greater than the total number of positive (normal or contra-) 

 valency electrons ; it is never greater than the sum of those 

 of class II. and class III. 



(b) The minimum number allowed by the dispersion and 

 magnetic rotation is never greater than the sum of those of 

 classes II. (a) and II. (b). 



(c) The maximum number of electrons of the longest 

 period allowed by the dispersion and magnetic rotation is 

 never greater than the number of electrons of class II. 



The empirical character of the four-constant formula 



2 1-. a ^ 



1=a » + ;?=v- d ' 



