n -~— , i.e. the refractive power of the dissolved substance d x
(Landolt and Wiillner, Schiitt).
More recently L. Lor em and H. Lorentz have proposed the formula ,— 5 — pr--„ which* is now almost universally (n 2 + 2) d J
accepted. It gives very constant values for the same liquid under different conditions, and seems even applicable to vapours, as shown by experiments on' ether, alcohol, chloroform, and water.
Molecular Refraction. — By multiplying the refrac- tive power of a substance by its molecular weight we obtain the molecular refraction. Using Lorenz and Lorentz's formula we have for the molecular refraction
?i 2 + 2* d'
The molecular refraction of a compound is composed additively of the refractive powers of the atoms contained in the molecule.
This rule being assumed, it is not difficult to ascertain, by empiric deductions, the atomic refractive powers of the principal elements contained in organic substances. Briihl has solved the problem in the following manner (n 2 formula and for the red hydrogen ray).
First of all he found that in all the homologous series of fatty compounds a difference of CH 2 in the composition corresponds to a difference of 4*57 in the molecular refraction. So that his first datum is : CH 2 = 4*57.
From the molecular refraction of an aldehyde or of a ketone (C n H 2n 0) he subtracts n times the value of CH 3 , and thus finds the constant 2*828 for intra-radical oxygen
(O").
By subtracting the molecular refraction of the hydro- carbon C n H 2n+2 from that of the corresponding alcohol CnH^+aO, he obtains for extra-radical oxygen (0') 1*506; and the same constant can be deduced from the molecular
�� �
