FLUIDITY AND TEMPERATURE

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temperature is raised, amounting to 4 per cent for a pressure of
400 atmospheres at 100°.
Faust (1913), using pressures as high as 3,000 kg per square
centimeter, found that the viscosity of ether, alcohol, and carbon
disulfide were each increased by about fourfold. This result
has important bearing upon the theory and practice of lubrica-
tion. And very recently J. H. Hyde (1920) has reported to
the Lubricants and Lubrication Committee of the Department of
Scientific and Industrial Research the results of an investigation
of the viscosity of a variety of lubricating oils, using pressures
up to 7 tons per square inch. He made the important deduction
that the mineral oils increase in viscosity far faster with the
pressure than do the fixed oils. Thus the viscosity of MobiUtl BB
increases over twenty-six-fold, whereas with the same increase
in pressure the fixed oils increase in viscosity about fourfold.
Fluidity and Volume.—We have now before us the two follow-
ing generalizations: (1) An increase in pressure is usually
associated with a decrease in fluidity, and (2) an increase in
temperature is usually associated with an increase in fluidity.
To be sure, there are prominent exceptions to both generaliza-
tions, as, for example, water, in its behavior under pressure and
sulfur, as affected by temperature. But water and sulfur are
highly associated in the liquid state so that an explanation of
these exceptions is possible on the basis of changing molecular
weights.
Lowering of pressure or raising of the temperature of a liquid
have one thing in common in addition to their similar effect
upon the fluidity—they both produce an increase in the volume,
to which there are very few exceptions. It is worth while there-
fore to investigate the question of how much of the change in
fluidity can be attributed primarily to a change in volume. If
one has in mind the fact that in gases, where the volume changes
are large, the fluidity is nearly independent of the volume, one
would naturally expect the changes in the volume of liquids to
be responsible for only a small part of the fluctuations in fluidity
which actually exist. But the viscosities of gases and liquids
arise from entirely different causes, hence reasoning by analogy
is useless.
The parallelism between fluidity and volume may be followed