4 FLUIDITY AA'D PLASTICITY
of The study of elasticity has been the dominant idea
in of researches.
Very little work has been done upon the viscosity of matter
in the states of aggregation taken as a whole. If it
has. been shown that our knowledge of viscosity consists of
unrelated groups, it is equally apparent that such a
is artificial and that nothing could be more important
for our understanding of viscosity, than to bring these
into an inter-related whole. We shall therefore
not an attempt to follow the chronological method, where
it with the consideration of the subject as a whole.
the groups of researches to which we have alluded
out rather clearly. The methods of measurement in use
will be first considered, after which we shall study the viscosities
01 solutions, solids, and gases respectively.
Deformation, Plastic, Viscous, and Turbulent Flow.—
If a perfectly elastic solid be subjected to a shearing stress a
strain is developed which entirely disappears when the
is removed. The total work done is zero, the process is
and viscosity can play no part in the movement.
is not a of flow but of elastic deformation. If a body
is imperfectly elastic as regards its form be subjected to
it will be found that a part, at least, of the
will remain long after the stress is removed. In this
has been done in overcoming some kind of internal
We may distinguish the kinds of flow under three
It is characteristic of viscous or linear flow that the
of deformation is directly proportional to the deforming
the ratio of the latter to the former gives a measure
erf It has been questioned at times whether this ratio
is but it appears that only one qualification is
In very viscous substances time may be necessary
for the to' reach a steady state, aside from any period of
because with substances like pitch the viscous
develops slowly, so that the above ratio gradually
when the load is first put on, but even in this case the
reaches a value which is independent of the amount
of tie As, however, the deforming force is steadily in-
m point may be reached where the above ratio suddenly