122

FLUIDITY AND PLASTICITY

to association but to want of sufficient data for calculating accu-
rately the atomic 'constants' and also to constitutional effects,
such as the mutual influence of groupings in the molecule, sym-
metry and so forth." As was intimated earlier in this chapter,
to chemical constitution has generally been attributed a very
large effect on viscosity, but it often turns out on investigation
that this supposed constitutive influence occurs in substances
that are known to be associated and this association was not taken
into account, and in other cases the supposed constitutive influ-
ence is almost certainly purely a hypothesis framed to explain
an unnoticed defect in the method of comparison. We shall now
give some facts to support these bare statements and we shall
then investigate the important question as to whether this dwind-
ling constitutive effect, as distinct from the effect of association,
can safely be disregarded altogether.

In assigning values to the halogen atoms, Thorpe and Rodger
(p. 669 et seq.) found it necessary to give a different value to
chlorine in monochlorides, dichlorides, trichlorides and tetra-
chlorides, but even then the results are not satisfactory since in
ethylene and ethylidene chlorides the value which must be
assigned the chlorine atom is certainly different. How the effect
of the chlorine atom varies at the fluidity of 200 is shown in the
fourth column of Table XXXII.

TABLE XXXII.—THE VALUE OF THE CHLORINE ATOM

Substance
Absolute temperature (<£ == 200), observed
Hydrocarbon residue, calculated
Chlorine
Association

Propyl chloride .
261 5
127 3
134 2
1 105

Isopropyl chloride ..... Isobutyl chloride.
255.2 285 2
119.7 142 4
135.5 142 8
1.11 1 13

Allyl chloride
256 0
123 3
132 7
1 10

Ethylene chloride .
336 5
45 4
145 5
1 27

Ethylidene chloride.. . . Methylene chloride. . . . Chloroform
291.2 279.1 " 305 3
45.4 22.7 - 36 5
122.9 128.7 113 9
1.10 1.15 1 04

Carbon tetrachloride . . Carbon dichloride .....
347.0 356.3
- 95.7
- 77.0
110.7 108.3
1.01 0.99