VISCOSITY AND FLUIDITY 83
great majority of cases it is sagged and there is no known law to
account for the peculiarity. Surely any discussion of chemical
combination or of dissociation on the basis of deviation from the
"normal ''curve under such conditions would be of very uncertain
"value.
There are numerous reciprocal relations besides viscosity and
fluidity, such as electrical resistance and conductance, or specific /
heat and heat capacity, or specific gravity and specific volume. *
It has been repeatedly pointed out1 that if one of these is additive, ,f/
its reciprocal cannot be. It is singular enough that among all I
of these reciprocal relations, viscosity is the only one for which ^
the decision has not been reached as to whether viscosity is fe
additive or not, or if it is, under what conditions. In electricity ]
for example we have absolutely no doubt but that resistances £,
are additive under certain conditions, viz., when the conductors |
are in series, and likewise that conductances are additive under |
other equally definite conditions, viz., when the conductors J
are in parallel. It seems probable that the present unsatisfactory , |
condition as regards viscosity has arisen due to the extraordinary f
sensitiveness of this property to molecular changes in fluids, f
either combination or dissociation. We shall attempt to reach f
a solution of the problem from a consideration of the nature of f
viscous flow and then test this solution by means of the experi-
mental facts. After we have reached a conclusion in regard to the
true additive property under given conditions, it may well turn
out that the present unsatisfactory condition will prove to be a
blessing in disguise, for it may then be shown that viscosity is of
the greatest importance in physiochernical investigations.
The fundamental law of viscous flow
*! =^
* ~ rj
is the analogue of the well-known electrical law of Ohm. In
fact Elie in 1882 suggested a modification of the Wheatstone's
bridge method for the measurement of viscous resistance.
Case I. Viscosities Additive—Emulsions.—We will first con-
sider the very simple case of a series of vertical lamellae of viscous
material arranged alternately, as in Tig. 32, and subjected to a
1 CJ. p. 89.