A Manual of the Chemistry of the Carbon Compounds

THE CARBON COMPOUND!*. 41
sin i
—.— = n
sair
The •value of n is called the index of refraction, and by speaking of
the index of refraction of a body, we mean generally, the ratio existing
between the sines of the two angles, when a ray enters the body
from the air. The index of refraction changes with the temperature
as well as with the specific gravity (d) in snch a way that the value
—j—, called the specific refractive energy, remains constant. By multi-
plying this value by the molecular weight of the substance, we obtain
its molecular refractive energy, which depends on the chemical composition.
The relations existing between the molecular refractive
energy and the chemical composition have been best studied for
the liquid compounds belonging to the group of fatty substances, and
the nou-snturated compounds associated with them.
The following are the most important xesnlts:—
(1) Isomerides and metamerides have the same molecular refractive
energy.
(2) The molecular refractive energy increases in homologous series
7-6, for each increase of CH2.
(3) By the addition of two atoms of hydrogen the molecular
refractive energy increases 2-6.
(4) The refractive molecular energy* of the acids of the series
CnHjnO2 is equal to 7-6«.+ 6.
From these results we can calculate the atomic refractive energy of
carbon, hydrogen, and oxygen; we find that in these liquid compounds
these elements have the following refractive energies:—
Carbon . . . . . . . 5
Hydrogen 13
Oxygen 3
By means of these members the molecular refractive energy of a
liqnid of known composition can easily be calculated* The following
example shows how well the value thus obtained agrees with that
fonnd by means of the observed index of refraction. To find the
latter, a hollow prism consisting of glass plates was filled with the
liquid, and by means of it, the deviation of the yellow sodium line
observed; the temperature being 20°.
Ethyl Alcohol C2H0O. d « 07964 n = 1-3606
—•^ = 0-4528.
By multiplying the latter number by 46, the molecxilar weight of
alcohol, we obtain the molecular refractive energy = 20-8, whilst
calculating it from the chemical formula we have—
2x5+6x1-3 + 3 = 20-8.