238 FLUIDITY AND PLASTICITY
more rapid rate of flow at first than later when the strains were
developed to their maximum amount, so it is common experience
that metals become harder with working, but that they may be
softened again by annealing. In the process of annealing, the
plasticity is increased by raising the temperature and thus the
strains relieve themselves more quickly than otherwise would be
the case.
An entirely different view from that given above has been
presented by Lord Kelvin and it has had many followers. Noting
that the logarithmic decrement of the vibration is greater in lead
and zinc than it is in steel, he reasoned as follows:
"Hence, there is in elastic solids a molecular friction which may be
properly called viscosity of solids, because as being an internal resistance
to change of shape depending on the rapidity of the change, it must be
classed with fluid molecular friction, which by general consent is called
viscosity of fluids.'J
However, he further stated:
" But at the same time it ought to be remarked that the word viscosity,
as used hitherto by the best writers, when solids or heterogeneous semi-
solid-semi-fluid masses are referred to, has not been distinctly applied to
molecular friction, especially not to molecular friction of a highly elastic
solid within its limits of high elasticity, but has rather been employed to
designate a property of slow continual yielding through very great, or
altogether unlimited, extent of change of shape, under the action of
continued stress."
It has thus come about that the logarithmic decrement has
been taken as a measure of the viscosity of a metal, so that
according to this nomenclature lead has a higher viscosity than
steel and the viscosity of lead increases as the temperature is
raised, which point of view is just the opposite of that used
by Kupffer and to which we are generally familiar in discussing
the viscosity of fluids. Since, however, several investigators have
followed Lord Kelvin in his nomenclature, there is danger of
considerable confusion. If we hereafter refer to the friction and
mobility of solids, the term "viscosity of solids" becomes
unnecessary; and we may confidently expect that the friction
constant of lead will be found to be lower than that of steel and
that it will decrease with the temperature.