FLUIDITY AND THE CHEMICAL COMPOSITION

115

1. The slopes of the fluidity-temperature curves for a given
homologous series are more nearly the same when the fluidities
are equal.

2. When the fluidities are the same, the vapor-pressures are
nearly equal, and experience has shown that substances are
comparable at temperatures which correspond to equal vapor-
pressure.

3. The fluidity curves of associated substances like the alcohols,
Fig. 46, depart widely from linearity at low fluidities, although
they approach linearity at high fluidities, as do the curves of
other compounds.

4. A yet more cogent reason grows out of the fact that exact

•JOO

200

50°

J50°

plo 45,—The fluidities of various organic acids at different temperatures.
48. Formic acid; 49. Acetic, acid; 50. Propionic acid; 51. Butyric acid; 52.
Isobutyrin acid.

parallelism in the curves of a given class is not to be expected
since all fluidity-temperature curves must undoubtedly meet at
the absolute zero of temperature. Hence while it may require
a constant increment of temperature to produce a given fluidity
as each methylene group is added to the molecule, it is absolutely
certain that a constant decrement of the fluidity at a given tem-
perature cannot be expected as each methylene group is added.
Thus a methylene group added to pentane, Fig. 37, lowers the
fluidity at 0° by a certain amount, but the effect of adding a