270 Mr. William Sutherland on the 



for present purposes to notice that the dynic equivalents given 

 for various radicals in Table XXVI. are closely proportional 

 to their molecular refractions. 



12. Close parallelism between Dynic Equivalents and Mole- 

 cular Refractions. — As is well known, there are two methods 

 according to which the molecular refraction is estimated, the 

 first by means of Gladstone's expression, (n — l)M//>, where n 

 is index of refraction ; the other by means of Lorenz's, 

 (n 2 -l)M/(n 2 + 2)p. 



In a brief paper (Phil. Mag. Feb. 1889) I showed that the 

 experimental evidence taken as a whole is in favour of the 

 Gladstone expression, for which also a very simple theoretical 

 proof can be given ; and, further, it was shown that it is best 

 to measure (w — l)M//o if possible in the gaseous state. But 

 as comparatively few measurements have been made on 

 bodies in the vapour state I suggested that, as the Lorenz 

 expression had been empirically proved to give more nearly 

 the same value in the liquid and vapour states of a body, its 

 value as determined in the liquid state and multiplied by 3/2 

 could be taken as giving the value of (n — l)M/p in the vapour 

 state. The result of the theoretical argument was that, if 

 M//o is taken to measure the molecular domain u, and if U is 

 the volume occupied by the molecule in the same units, and X 

 is the index of refraction for the matter of the molecule, then 



[n-l)u = (X-l)U. 



Landolt, Briihl, and others have determined the values of the 

 atomic refraction for several elements (Ann. tier Chem. ccxiii. 

 p. 235), and by means of these and Masini's data for sulphur 

 (Wied. Beibl. vii.) and Gladstone's latest determinations 

 (Journ. Chem. Soc. 1884), I have obtained the values of the 

 refraction-equivalents of the preceding radicals in terms of 

 that for CH 2 as unity. Mascart has given (Compt. Rend. 

 lxxxvi.) values of the refraction of a number of substances in 

 the vapour state, from which, for the sake of comparison, I 

 have calculated the refraction-equivalents for as many radicals 

 as possible. 



The following Table contains in the second column the 

 dynic equivalent, in the third the refraction-equivalent calcu- 

 lated according to the Lorenz expression, in the fourth the 

 refraction-equivalent calculated according to the Gladstone 

 expression from Mascart's data for vapours, and in the fifth 

 that calculated by Gladstone from liquid data. The value for 

 CH 2 in every case is 1. 



