144

FLUIDITY AND PLASTICITY

If we could always compare the fluidities of liquids at a definite
multiple of the molecular limiting volume, it is evident that the
effect of the volume, and therefore of temperature and pressure
in so far as they affect the volume, would be eliminated. Such a
procedure would enormously simplify fluidity relationships.
As a matter of fact Batschinski has shown that the molecular
limiting volume possesses an additive character, the values of

50 Fluidity" _550
FIG. 55.—Relative volume-fluidity curves after Batschinski using the data of
Thorpe and Rodger.
4, Bromine; 5, nitrogen peroxide; 9, isohexane; 11, isoheptane; 13, trimethyl-
ethylene; 14, isoprene; 15, diallyl; 16, propylchloride; 17, isopropylchloride;
18, isobutylchloride; 19, allylchloride; 20, methylenechloride; 21, ethylenechlo-
ride; 22, ethylidenechloride; 23, chloroform; 25, perchlorethylene; 68, ethyl-
benzene; 69, orthoxylene; 70, metaxylene; 71, paraxylene.
the atomic constants being H = 4.3, 0 = 8.6, C = 8.8, Cl =
19.2, Br = 24.8, I = 32.0, S = 19.0, a double bond = 3.3,
and an iso-grouping = 0.7. For 53 of the substances studied by
Thorpe and Rodger the differences between the observed and
calculated values of the molecular limiting volume do not exceed
2 per cent. The limiting specific volume is approximately 0.307
. of the critical volume which is close to the parameter b of van
der Waals' equation.
We are now in a position to get a very clear understanding
of the law that the fluidity is proportional to the free volume.