flay vil 
XIII. 
ON THE LAW OF GLADSTONE AND DALE AS AN 
OPTICAL PROBE. By PROFESSOR W. J. SOLLAS, 
Wascen Ul. Bons. 
(Abstract of a Paper read January 18, 1893, and to be published in extenso in the 
Screntiric TRANSACTIONS OF THE Royat Dustin Society, Vol. V.) 
Tue law of Gladstone and Dale asserts the constancy of the ratio 
of the refractive index to the density for all substances, indepen- 
dent of the physical state in which they exist. It is expressed by 
the equation i= = «x, where m signifies the refractive index, 
d the density, and « a constant known as the specific refractive 
energy. If the specific refractive energy of any substance be 
multiplied by the molecular weight (m), a number is obtained 
known as the refractive equivalent, which may be indicated by 
the Greek letter A. 
If, as verified by experiment, the elements retain their refrac- 
tive equivalents unchanged in the state of chemical combination 
the refractive equivalent of a compound will be the sum of the 
refractive equivalents of its elements. Thus in the case of 
sodium oxide we have 2Na, +O, = Na,O,. Hence, given the 
refractive equivalents of the elements, we can find the specific 
refractive energy (x) of the compound: for, calling the molecular 
weight m, we have: mx = Na,O,, and consequently, 
7 Na.O, re 2Naqa + O7% 
m m 
K 
Again, since —_ =k, given « and d, we can find the refrac- 
tive index, or given «x and the refractive index, we can find the 
density. 
