156

FLUIDITY AND PLASTICITY

the addition of a methylene group causes a decrease in the fluidity
A which is AB or CD. It appears that AC/AB = CE/CD, &
the ratio between the effect produced on the boiling-point to th<*
effect produced on the fluidity F/A is constant, for this particular'
homologous series. This relation does not apply to the alcoholH
and acids. A reason is that as the temperature is raised, th<-
association is lowered, F becomes smaller and at the same time A
becomes greater, so that their ratio may vary.

Fluidity and Vapor Pressure.—If there is a relation betweer*
the fluidity and the boiling-point, it is evident that a more

500 400 300
eoo too
0

^r
^^r
^2*-
-------- ?$

J*
*r*^
o A
ethyl %/ P
!£°®
Propyl ropyll ///the Ether
Ether Ether *r

/I

af x D
- n r

P
FIG. 58.—Fluidity-vapor pressure curves of a series of ethers.
general relation than the above can be obtained by comparison
at other vapor pressures than at the ordinary boiling temperature.
Thus if all of the aliphatic ethers have the same fluidity at the
ordinary boiling-point, and the same were true at other vapor
pressures, it follows that, when the vapor pressures corresponding
to a given temperature are plotted against the fluidities corre-
sponding to that temperature, and this process is repeated for a
series of temperatures, the curves of all of the substances of the
class should fall together; in other words, the fluidity vapor-pressure
curve of one ether ought to be the curve of all the other members
of the class. Conversely, if either the vapor pressure or the
fluidity of an ether is known for a given temperature, the other
quantity, supposedly unknown, can be determined by means of