276

FLUIDITY AND PLASTICITY

fluidity of the paraffin is strikingly shown by introducing a
paraffin residue (CH3) into the benzene ring, which results in
toluene (C6H8) having a higher fluidity than benzene (C6H6).
On the other hand, toluene has a much lower fluidity than the
purely paraffin compound heptane (C7Hi4) which contains the

100 ?00

800 900 1000

300 400 500 600 700
Vapor Pressure in mm
FIG. 89.—Fluidity-vapor-pressure curves of hydrocarbons of different homolo-
gous series. (Of. Fig. 58.)
same number of carbon atoms. It may be urged that whereas
these compounds contain the same number of carbon atoms they
do not contain the same number of hydrogen atoms. But one
should also note that diallyl contains more hydrogen atoms than
benzene and less than hexarnethylene and yet has a fluidity
which is far higher than either. The cyclic compounds may owe
their low fluidity to association, but the relation of association
to the properties desired in a lubricant is not well understood.
However, the relation between fluidity and vapor-pressure,