138

FLUIDITY AND PLASTICITY

Fluidity and Pressure.—To find the effect of pressure on
viscosity, Coulomb in 1800 measured the rate of oscillation
of a disk in a liquid both under atmospheric pressure and when
the space above the liquid had been evacuated. It was found
that the viscosity is independent of small changes of pressure.
This conclusion was confirmed by Poiseuille in 1846, using his
transpiration method.

However, quite the opposite conclusions must be drawn
from the experiments of Warburg and Babo (1882), but they
employed liquid carbon dioxide at 25.1°C, which is quite near the
critical temperature, and they used pressures from 70 to 105
atmospheres. It is worth noting that under these conditions
the compressibility of carbon dioxide is 0.00314 which is about
18 times as great as that of ether at the same temperature. They
found an increase in the viscosity which amounted to over 25
per cent, and it therefore seemed possible that the effect was
caused by the change in density and that a similar effect would be
observed in other liquids if high enough pressures were employed.

Warburg and Sachs (1884) continued the previous investigation
and indeed found that liquid carbon dioxide, ether, and benzene
all suffer an increase in viscosity on increasing the pressure, but
they also noted that water is exceptional in that an increase in
pressure actually lowers the viscosity. They sought to connect
the viscosity and pressure by means of the following linear
formula,

T? = .70(1 + Ap). (55)

The values of the constant A are given in Table XXXVI.

TABLE XXXVI.—CONSTANTS IN EQUATION (53)

Substance
Carbon dioxide
Ether
Benzene
Water

Temperature of experiment
25 1
20
20
20

Critical temperature
30 9
190
280.6
365

A X 106 .........
7,470
730
930
— 170

The striking fact that water is peculiar in this as in so many
other respects was discovered independently by Rontgen (1884).
It was made the subject of a special study by Cohen in 1892,