FLUIDITY AND TEMPERATURE 151
that diffusion between adjacent layers is comparatively slight,
whereas the volume of the molecules themselves is a consider-
able portion of the total volume of the liquid. Given a layer of
molecules C whose translational velocity is higher than that of
another layer D, there must of necessity occur collisions between
the two layers due to the flow, and quite irrespective of any
diffusion, provided only that the diameter of the molecules is
greater than the distance between the layers. On collision, the
translational velocity is partly communicated to the slower
moving molecules of the layer D, so that the molecules of the
layer D have a mean resultant velocity in the direction of the flow,
the remainder of the translational motion being converted' into
disordered motion or heat. When 'the system has reached a
steady state, any layer D imparts to the layer E below it the same
amount of translational momentum that it has received from the
layer C above it, except for the amount of energy which is being
continuously changed into heat, and it is this disappearance of
translational momentum which gives rise to the new type of
viscous resistance known as collisional viscosity. Since each
layer is able to transmit but a portion of the translational momen-
tum, which it receives to the adjacent more slowly moving layers,
there results a steady diminution in the velocity of flow from the
most rapidly moving layer A to the layer which is at rest B.
From this model of viscous flow in liquids it is possible to deduce
the effects of changes in concentration, pressure, temperature, and
size of the molecules. The number of collisions of the particles
of one layer with those of another layer, due to translational
velocity, will be directly proportional to the rate of shear between
the layers. This is the fundamental law of viscous flow. It will
also be directly proportional to the number of molecules in each
layer, i.e. to the concentration. It is a confirmation of this
prediction, that we find that the fluidity is decreased in just the
proportion that the concentration is increased either by lowering
the temperature or by raising the pressure. It is significant
that the temperature by itself is without effect on collisional vis-
cosity. The reason for this is evidently that the mere vibration
of the molecules without diffusion through successive layers does
not affect the rate at which the molecules of one layer overtake
the molecules of an adjacent layer moving more slowly. It is