114

FLUIDITY AND PLASTICITY

curves are nearly straight and parallel lines. However as we get
away from the boiling-temperature, there is a curvature present
so that it is probable that the fluidity curve would reach the tem-
perature axis asymptotically as the temperature were lowered.
Broadly speaking, the curves of a given homologous series near
their boiling-points consist of a series of parallel straight lines,
which are therefore completely defined mathematically by their
slopes and intercepts. We find the same thing in other series,

1 fci-----'------5o*------'------m------' j35»

FIG. 43.—The fluidities of vari-
ous bromides at different tem-
peratures. 20. Ethyl bromide;
21. Propyl bromide; 22. Iso-
propyl bromide; 23. Isobutyl
bromide; 24. Allyl bromide; 25.
Ethylene bromide; 26. Propy-
lene bromide; 27. Isobutylene
bromide; 28. Acetylene bro-
mide.

,200

2.

25' so° 15° 100°

FIG. 44.—The fluidities of various
iodides at different temperatures.
14. Methyl iodide; 15. Ethyl iodide;
1C. Propyl iodide; 17. Isopropyl
iodide; 18. Isobutyl iodide; 19.
Allyl iodide.

as the ethers and acid anhydrides given in Fig. 42, but it is clear
that the slope is different in the two classes. The slope then is
dependent upon the class to which a compound belongs and the
intercepts are dependent upon the chemical composition.
According to this broad aspect of the case it should make no dif-
ference whether we compare fluidities at a given temperature or
temperatures corresponding to a given fluidity. But there are
several reasons for choosing the latter basis of comparison rather
than the former.