Mr. W. Sutherland on Molecular Refraction. 143 



between index and density retained its value as an empirical 

 physical law. 



In the course of some researches as to how far (n^—l)/d 

 could be regarded as constant for liquids when d is changed by 

 heat, Gladstone and Dale (Phil. Trans. 1858) discovered that 

 (7i—l)/d is more nearly constant than Newton^s expression. 

 Berthelot had drawn attention to the desirability of studying 

 for different substances not only {n^ — l)/d, but M{n^ — l)/d, 

 where M is the molecular weight. Landolt, doing this with 

 Gladstone's expression, discovered that the molecular refraction 

 of a substance, 'M{n—l)/d, is the sum of the definite atomic 

 refractions or refraction-equivalents of the elements composing 

 it. Subsequent researches by Gladstone showed that certain 

 elements have not a single definite refraction-equivalent, but 

 different equivalents according to their different methods of 

 chemical union with other elements. As this discovery pro- 

 mised to be of value to those chemists who are eager to take 

 advantage of all the light that physical investigations can 

 throw on chemical structure, its further prosecution was taken 

 up by Briihl, who established the important result for structural 

 chemistry that whenever two carbon atoms are connected 

 ethylenewise (or, in the " bonds '^ terminology, when a carbon 

 atom is united to another by a pair of bonds) the refraction of 

 the molecule is increased by a definite amount above the sum 

 of the equivalents of the atoms in the molecule. Kanonnikoff 

 was able, as a first-fruit of optical chemistry, to show how 

 Briihl's discovery throws immediate light on the debated 

 points of the comparative structure of citraconic, itaconic, and 

 mesaconic acids and of the different terpenes. 



A large amount of experimental work had thus been under- 

 taken, all on the strength of Gladstone's empirical law, and 

 no physical theory had been advanced to give the law meaning, 

 when Lorenz ot Copenhagen, and Lorentz of Amsterdam, 

 brought out each a mathematical theory of molecular refrac- 

 tion ; the former applying his treatment of the ordinary 

 undulatory theory, the latter using the electromagnetic hypo- 

 thesis, but both coming to the same result, but one quite 

 different in form and meaning from that of Gladstone's law. 

 They found that on these assumptions (n2 — l)/(?i^ + 2)fZ ought 

 to be constant for a given body. It would be impossible to 

 give in a brief form an analysis of their mathematical work as 

 presented in the German translations of their papers in Wie- 

 demann, volumes ix. and xi.; but a slight sketch of Lorenz's 

 method is necessary to make clear the following discussion 

 of the merits of his formula in comparison with that of 

 Gladstone. 



