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II. On the Refraction-Equivalents of the Elements. 
By J. H. Gladstone, Ph.E., F.R.S. 
Keceived June 17, — Read June 17, 1869. 
In our paper “ On the Refraction, Dispersion, and Sensitiveness of Liquids,” Mr. Dale 
and I pointed out a property of bodies which we termed their “specific refractive 
energy.” It is the refractive index minus unity, divided by the density, or in symbolical 
language We found that this is a constant unaffected by temperature, and that 
Cl 
the specific refractive energy of a mixture is the mean of the specific refractive energies 
of its constituents. At the same time, however, we admitted that in both cases our 
numbers were not in perfect accordance with theory, there being some unknown cause 
which affected them to a slight extent. These conclusions, both in regard to the general 
law and its qualification, have been since confirmed by continental physicists, and espe- 
cially by the late rigorous experiments of Wullner*. 
In the same paper we ventured also on the generalization that “ every liquid has a 
specific refractive energy composed of the specific refractive energies of its component 
elements, modified by the manner of combination.” Later research has confirmed this 
also, extending it to conditions of matter other than liquid, and showing more clearly 
when such modifications occur, and what is their nature. Professor Landolt, of Bonn, 
has greatly advanced our knowledge of the subject, and has simplified the calculations 
by adopting what he terms the refraction-equivalent, that is, the specific refractive energy 
multiplied by the atomic weight, or P Recent investigations in fact tend to the 
general conclusion that the refraction-equivalent, not only of mixtures, but of every com- 
pound body, is the sum of the refraction-equivalents of the elements that compose it. 
Were this perfectly true, like the statement “the atomic weight of a compound is 
the sum of the atomic weights of its constituents,” it would be a simple matter to deter- 
mine the refraction-equivalents of all the elements ; and then we should be in a posi- 
tion to calculate the effect of every transparent body of known composition on the rays 
of light transmitted by it. But it is not absolutely true : even in Landolt’s first 
paper it is evident that there are exceptions; the unknown cause which modifies the 
refraction of mixtures probably acts in cases of more perfect chemical combination ; 
and the conviction has grown that some elements have two or more refraction-equi- 
valents. 
I have continued from time to time to make observations on this subject, and tfie 
* Pogg. Annalen, vol. cxxxiii. p. 1. 
MDCCCLXX. 
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