246

FLUIDITY AND PLASTICITY

seems most reasonable to assume that the temperature is without
effect, in which case we should expect the diffusional viscosity
to vary directly as the square root of the absolute temperature.
Maxwell concluded from Ms experiments that the viscosity
varies directly as the first power of the absolute temperature.
Barus (1889) worked with air and with hydrogen over a very
wide range of temperature from 0 to 1,300° and found that the
viscosity increased as the two-thirds power of the absolute
temperature. Holrnan (1877) and (1886)) in a careful investiga-
tion of the subj ect had found the exponent to be 0.77 for air. On
the other hand, easily condensihle gases and vapors such as
mercury, carbon dioxide, ethylene, ethyl chloride and nitrogen
peroxide give values of the exponent which are nearly unity,
according to Puluj (1876) and Obermayer (1876); but E. Wiede-
rnann (1876) discovered that the value grows smaller as the tem-
perature is elevated, which we might have anticipated since they
thus become more nearly like the permanent gases. The vis-
cosity of many vapors increases even more rapidly than the first
power of the temperature. Schumann (1884) used the formula

= KT3A.

(99)

Sutherland (1893) believes that "the whole of the discrepancy
between theory and experiment will disappear if in the theory
account is taken of molecular force. * * Molecular attraction
has been proved to exist, and, though negligible at the average
distance apart of molecules in a gas, it is not quite negligible
when two molecules are passing quite close to one another; it
can cause two molecules to collide which in its absence might
have passed one another without collision; and the lower the
velocities of the molecules, the more effective does molecular
force become in bringing about collisions which would be avoided
in its absence.
" Molecular force alone without collisions will not carry us
far in the explanation of viscosity of gases as known to us in
nature, because in all experiments on the viscosity of gases there
is a solid body which either comnmnicates to the gas motion
parallel to its surface or destroys such motion, so that the mole-
cules of gas must collide with the molecules of the solid; for if the
molecules of gas and solid act on one another only as centers of