$B MOLECULAR ASSOCIATION
view; the result of experiment, however, may be said to prove
the correctness of the second.
The proof of the reduced value in the molecular weight ob-
served when combination occurs lies in the study of cases in
which combination is known to occur.* It has recently been
pointed110 out that, although the general effect of combination is
always to make the observed molecular weight lower than it
would be in the absence of such combination, there are five ways
in which the molecular weight may vary with concentration,
supposing molecular association and combination both to exist.
1. The molecular weight decreases with increasing concentra-
tion.—Benzoic acid in/-toluidine,117 for example, where combina-
tion occurs, illustrates this case. Another example, of a different
type> is that of lithium chloride in ethyl alcohol.118 These two
substances combine to form LiCl, 4C2H0O. The variation of the
molecular weight is as below, C indicating the number of grams
of the salt dissolved in 100 grams of the alcohol:—
• c. M.W.
2-244 34-0
2-655 33-0 Lid = 42-5
3-406 30-1
4*075 29-2
Hydrogen chloride, both in benzene and in nitrobenzene, is
strongly associated, but as the concentration increases, the ap-
parent degree of complexity falls off, a fact only to be explained
by the occurrence of chemical combination.119
2. The molecular weight increases with increasing concentra-
tion.—Here, despite combination, the rate of increase of com-
plexity is greater than the reducing effect due to combination.
Several examples occur in the determinations of Werner, such as
in the case of cadmium bromide in piperidine.120
3. The molecular weight increases to a maximum and then
falls.—Such a phenomenon will be observed in cases where there
is a rapid increase in the degree of association in the early stages
* Neglect to take into account the heat of dilution introduces the same kind of
an error as is now discussed under the head of chemical combination. Thus, when
the heat of dilution is taken into account, the molecular weights of sodium and potas-
slum in mercury (p. 23) have normal values. It is probable, however, that in most
cases where it is necessary to correct the osmotic pressure (freezing-point, etc.), for
heat of dilution, that chemical action between solute and solvent occurs; so that a
chemical explanation can, in fact, be offered for an abnormal physical property.
For a discussion of the effect of heat of dilution see Fincllay's " Monograph on
Osmotic Pressure " in this series.