FLUIDITY AND VAPOR PRESSURE 159
when they belong to very different classes. For 17 substances
compared by this method, the average deviation between the
observed and calculated values is approximately 3 per cent,
so long as the vapor pressure is above 10 cm.
A few examples will serve to make the use of the method clear.
Thus the fluidity of ethyl acetate at its boiling-point (77.2°C)
is 395.6 and the reduction factor is therefore 500/395.6 = 1.264.
The fluidity of the substance as determined by Thorpe and
Rodger for 30° is 249.9 and the reduced fluidity is therefore
249.9 X 1.264 = 315.8, and the vapor pressure read from the
Table XL for the standard curve corresponding to this
fluidity is 120.4 mm, while the vapor pressure observed by Young
is 118.7 mm. As a further example, ethyl propyl ether has a flu-
idity of 479.9 at its boiling-point (63.4°). The factor is 500/479.9
= 1.042. The fluidity at 20° as determined by Thorpe and
Rodger is 314.9 and this reduced is 314.9 X 1.042 = 328.1, and
according to the Table XL this corresponds to 142.5 mm which
is practically identical with the experimental value of 142.6.
Associated substances do not show the relation between
fluidity and vapor pressure shown elsewhere. The greatest
deviation is shown by isobutyl alcohol and formic acid and ethyl
alcohol. In Fig. 59, there is plotted the vapor-pressure-tempera-
ture curve of heptane (not reduced) with several "associated"
substances which show large deviation from the standard. We
note that isobutyl alcohol, formic acid, and ethyl alcohol show
the most rapid increase in the vapor pressure. This is added
evidence of the breaking down of association. (C/. pp. 276
and 277.)
Before concluding our consideration of vapor pressure it may
be remarked that since fluidity is related to volume and at the
same time to vapor pressure, there is necessarily a relation be-
tween volume and vapor pressure. The volume is doubtless
affected by the internal forces between the molecules which we
ordinarily call cohesion and vapor pressure naturally depends
upon the same. So there may quite possibly be a connection
between fluidity and cohesion, even though it is not the connec-
tion which is often supposed.1
Cf. p. 147 et seq.