78 OUTLINES OF PHYSICAL CHEMISTRY
light wave in the air and in the liquid, then it follows that
��w-l =
��v — v x
��that is, it expresses the ratio of the excess of velocity in air over velocity in the liquid to the velocity in the liquid.
2. If we accept the theory of dielectrics of Clausius and Mosotti,Mid the electromagnetic light theory (Maxwell),
we find that the factor a ~~ expresses what fraction of
the unit of volume of the substance is really occupied by the material. The molecular refractive power
n 2 — 1 m
��ri l
��+ 2* d
��would therefore only be a corrected molecular volume — the
material volume occupied by the atomic nuclei of the
molecule (Exner, Berichte der deut. chem. Oesell. 18. B.
355 ; Briihl, ibid. 19. 2755).
3. There is an approximately constant ratio between
this corrected molecular volume and the sum of the atomic
volumes calculated by Traube's method (Berichte, 29.
2782).
2 atomic volumes
��n % — 1 m
��= about 8*45.
��This would, therefore, be the ratio between the oscilla- tory volume of the atoms and their real (nuclear) volume.
The presence of double linkages causes a diminution in this ratio. In order to observe any regularity it is necessary to use the maximum values found by Traube for the atomic volumes — thus for oxygen 5*5 must be taken.
4. For the molecular and atomic dispersion of light, reference should be made to J. W. Briihl's publications in the ' Berichte der deutschen chemischen Gesellschaft ' and in the ' Zeitschrift fur physikalische Chemie ' (since 1887).
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