It is beyond the scope of this work to treat thoroughly of molecular refraction.27 However, a brief discussion of the fundamental concepts involved may prove useful.
The index of refraction of a liquid varies with the temperature and with the wave length of the light. In order to obtain a constant which is independent of the temperature, Gladstone and Dale28 introduced the use of "specific refractivity." Subsequently, Lorentz,29 and Lorenz30 independently deduced an expression for specific refractivity, based upon the electromagnetic theory of light, which shows considerably less variation than the empirical expression of Gladstone and Dale. In order to compare the refractivities of different liquids, the use of molecular refractivity (molecular refraction) is necessary. This constant is equal to the product of the molecular weight of a substance and its specific refractivity.
27 Reference may he made to the original papers of Eisenlohr, of Swientoslawski, and of Brtthl and to any standard text on physical chemistry for further discussion of this interesting
physical property.
28 M Roy. Sor. London, Phil Trans. 148, Part I (1858), 887,
29 Ann. Physik Chem. N.S. 9 (1880), 641,
30 Ibid. 11 (1880), 70, *

Using the Lorentz and Lorenz expression : 
Using the Lorentz and Lorenz expression

The molecular refractivity has been found to be essentially additive. Hence, it is possible to calculate atomic refractivities for the different elements from a series of molecular refractivities of different compounds. By means of these atomic constants, the molecular refractivity of a pure chemical compound can be calculated as the sum of the atomic refractivities.

Investigation has shown, however, that the molecular refractivity is influenced by the presence of double and triple bonds, and also by the constitution of the molecule. Table 4.6 gives values for atomic refractivities for the D line of the solar spectrum (sodium light), 5893 angstrom units, calculated by different investigators. By use of these constants it is often possible to establish or confirm the chemical constitution of a pure chemical compound.
Certain anomalies have been observed. When double bonds are present in a conjugated position, the molecular refractivity will show in general a higher value than one would expect; this is known as optical exaltation. In some cases optical depression is also encountered. It is interesting to note that conjugated double bonds in a ring compound cause no exaltation or depression.
The application of molecular refraction is limited to pure individual chemical compounds; it becomes meaningless when applied to mixtures as complex as essential oils. Nevertheless, this constant has played a very important role in the elucidation of structure in the case of many individual constituents of essential oils after separation and purification.

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