METHODS OF MEASUREMENT 3
The rise of modern physical chemistry resulted in an awaken-
ing of interest in all of the properties of aqueous solutions.
Along with other properties, viscosity received attention from a
great number of physical chemists, among whom we may cite
Arrhenius (1887), Wm. Ostwald (1893), J. Wagner (1883-90),
Reyher (1888), Mutzel (1891). It must be admitted that our
knowledge of viscosity has not played an important part in the
development of modern physical chemistry. It is doubtless for
this reason that the subject of viscosity is left unconsidered in
most textbooks of physical chemistry. It is certainly not be-
cause viscosity does not play an important role in solutions, but
rather that the variables in the problem have not been properly
estimated. That with the physical chemist viscosity has so
long remained in the background, makes it all the more promis-
ing as a subject of study, particularly since it is becoming more
and more nearly certain that viscosity is intimately related to
many very diverse properties such as diffusion, migration of
ions, conductivity, volume, vapor-pressure, rate of solution and
of crystallization, as well as chemical composition and consti-
tution, including association and hydration. It seems probable
that the ^ork in this field is going to expand rapidly, for it is
becoming imperative that the exact relation between viscosity
arid conductivity, for example, should be clearly demonstrated.
With the recent advances in our knowledge of the nature of
colloids, there was certain to be an extended study of the vis-
cosity of these substances, because no property of colloids is so
significant as the viscosity. This in turn has again stimulated
interest in viscosity on the part of the physiologist, so that the
viscosity of blood, milk, and other body fluids have been
repeatedly investigated under the most varied conditions during
the past few years.
The use of viscosity measurements for testing oils, paints,
and various substances of technical interest has given rise to a
series of investigations, that of Engler (1885) being among the
"earliest and most important in this group. These researches
have been devoted largely to devising of instruments and to a
comparison of the results obtained.
Quite unrelated to the above groups for the most part, are
the investigations which have undertaken to study the viscosity