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FLUIDITY AND PLASTICITY

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are apparently examples of this class. Gurney (1908) in investi-
gating the contamination of pure water surfaces on standing,
says " Water surfaces become noticeably rigid in a few hours or
days: depending on the previous history of the fluid. Vigorous
stirring destroyed the rigidity of the surface."
To prevent possible misunderstanding, it must be stated
again that rigidity in foams and emulsions arises largely from
the fact that during shear the bubbles of a foam or the
globules of an emulsion are distorted and may be disrupted,
and thus work is done against the forces of cohesion opposing
such disruption.
Superficial viscosity has heretofore been considered at a free
surface only. Such a view is too narrow as it would leave the
most important examples out of consideration and from the
theoretical aspect the extension of our conception of superficial
fluidity involves no difficulty whatever. Having made this
extension, the phenomenon of slipping falls into the third case,
but the fluidity near the boundary is higher than that of the main
body of material. Henry Green (1920) has studied this slippage
under the microscope, using for observation paint colored with a
little ultramarine, which may be subjected to shearing stresses
in a capillary tube. With small stresses the shear takes place
exclusively in the region near the boundary, but when the stress
becomes greater than the yield value of the paint, the shearing
takes place throughout the material. Green reasons that it is
this mixture of the kinds of flow which causes the shear to fail
to be a linear function of the shearing stress, particularly when
those stresses are near the yield shearing stress. In the above
example, the layer next to the boundary was more fluid than the
main body of material, but more often the opposite is the case, the
fluid near the boundary is less fluid, and we might therefore consider
the general subject of adsorption under this head. And we would
then show that it is possible to make a fractional separation of
fluids by simply passing them through capillary tubes. Such a
separation of a mixture into its components by means of capillary
flow has actually been demonstrated, as in the case of petroleum
forced through clay by Gilpin and his co-workers (1908).1 Since
the surface area of a capillary varies as the first power of the
^ Am. Chem. J. 40, 495 (1908); 44, 251 (1910); 60, 59 (1913).