242 FLUIDITY AND PLASTICITY

viscosity is the simplest to obtain and it may be used to calculate
the other two, so viscosity played an exceedingly important part
in the years that followed in the establishment of the kinetic
theory on a firm basis. Maxwell defined the unit of viscosity;
and the theory of viscosity and its measurement was rapidly
advanced by Maxwell, 0. E. Meyer and many others. After
many vicissitudes, the conclusion was reached that viscosity
is a fundamental property independent of the particular method
used in its measurement. Thus, for instance, Millikan (1913)
brought together the results for air at 23° by five different
methods and found them t'o agree to within less than 0.1 per cent
as given in Table LXV.

TABLE LXV.—THE VISCOSITY OF AIR AT 23°C (FROM MILLIKAN)
0.00018258 Tomlinson...........Damping of the swinging of a

pendulum................. (1886)

0.00018229 Hogg................Damping of an oscillating

cylinder......'............. (1905)

0.00018232 Grindley and Gibson.Flow through a large tube..... (1908)

0.00018257 Gilchrist.............Method of constant deviation.. (1913)

0.00018227 Rapp................Transpiration method......... (1913)

0.00018240 Average value
Between 12 and 30° the viscosity of air is given by the following
formula with an accuracy of nearly 0.1 per cent according to
Millikan:
rit = 0.00018240 - 0.000000493 (23° - t)
The reader may, however, be referred to the more recent paper of
Vogel (1914).
THE THEORY OF THE VISCOSITY OF GASES
The theory of gaseous viscosity has been so often stated
that it need be stated here only in the simplest terms. The
viscosity of a gas is given by the tangential force required per
unit area to maintain a unit velocity in a plane of indefinite
extent at a unit distance from another parallel plane supposed
to be at rest, the space between the planes being occupied by the
gas. It is assumed that if the shearing force is equal to the vis-
cosity, the velocity v at any point will be numerically equal to its