CHAPTER IV
; FLUIDITY AND VAPOR PRESSURE
/
All physical and chemical properties will perhaps in time
be shown to be related so that the knowledge of a certain set of
facts in regard to a substance, such for example as its chemical
structure, will enable one to deduce it multitudinous properties.
Thus having established a direct causal dependence of the fluidity
1 upon the volume, it is also important to study other properties
! which depend upon the fluidity, or which together with the
fluidity depend upon a common cause. Migration velocity and
| electrical conductivity of solutions are examples of properties
i which are directly dependent upon the fluidity. There are
properties which are not dependent upon the fluidity directly but
which with the fluidity are dependent upon the same property
and therefore are indirectly related. The boiling-point, the
critical temperature and the vapor-pressure are properties of this
«, latter type, which we will now consider.
I Fluidity and Boiling-point.—On examination of the fluidity-
temperature curves of the aliphatic hydrocarbons, Fig. 41, and
ethers, Fig. 42, we note that the fluidities of these substances at
their boiling-temperatures—shown by small circles—are nearly
identical. It is perhaps of no Special significance that the flu-
idities are identical, but it is important that the line connecting
the fluidities at the boiling-points is linear. This linear character
of the fluidity-boiling-point curve is exemplified by the aliphatic
chlorides, bromides and iodides as well as by the ethers and
hydrocarbons. The acids and alcohols are again exceptional.
The meaning of this relation may be most easily grasped
by reference to Fig. 42. If we assume that the curves of the
members of a given class have the same slope and the same
degree of curvature at the boiling-point, it is evident that the
addition of a methylene group to a molecule causes a rise in the
boiling-point F measured by AC1 or CE, but at the same time
1 <?/. Fig. 42.