FLUIDITY AND TEMPERATURE 147
to the familiar pressure-volume diagram. At the highest
pressures, the fluidity is not greatly affected by a change in
pressure, e.g., at 32° and a pressure of 120 atmospheres, a lowering
of the pressure by 4 atmospheres causes an increase in fluidity
of less than 4 per cent. At a lower pressure the fluidity becomes
extremely susceptible to changes in pressure, a lowering of the
pressure by 4 atmospheres at 76 atmospheres causing an increase
in the fluidity of a full 100 per cent at 32°. The gaseous and liquid
phases are both present inside of the curve kbmcl. But the right side
of the figure is entirely different from the familiar pressure-
volume diagram. Instead of the fluidity being highly susceptible
to changes in pressure, as is the volume, it is but slightly affected,
e.g., at 32° and 50 atmospheres pressure, a lowering of the pressure
by 4 atmospheres causes only a 10 per cent increase in the fluidity.
Let us follow in detail the isothermal of carbon dioxide at 20°
which is well below the critical temperature. At high pressures,
the fluidity increases nearly linearly from a to 6; there is then
a sudden increase in the fluidity from 1,500 to 5,300 absolute
units, as the substance passes from the liquid to the gaseous
condition. We should expect the fluidity to continue to increase
as the pressure is further lowered, giving the curve cd', but the
curve actually obtained is cd. We have seen that the fluidity of
liquids increases with the temperature, while, on the other hand,
the fluidity of gases decreases with the temperature, hence, the
pressure-fluidity curves for different temperatures must intersect
each other. The figure proves that not only is this true, but, -
when the temperatures are sufficiently high, the curves all tend to
pass through the particular point n, so that at this point the
fluidity is independent of the temperature; for the lower tempera-
tures, the curves seem to intersect each other on the curve ncL
Collisional and Diffusional Viscosity.—That the pressure-
fluidity curves do not follow an equation of the van der Waals
type as the fluidity becomes large may be due to the appearance
of a new type of viscous resistance. We must therefore now
investigate more particularly into the nature of viscous resis-
tance. One's first impulse in looking for a cause- of viscosity is to
assume a cohesion between the particles which is exerted during
motion and acts in opposition to motion/ but with the develop-
1 "Kinetic Theory of Gases," MEYER, p. 171.