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114 OILINESS

and Lewin claim similar results from ex in draining capillary tubes. Bastow andfi^lments however claim that such data are, for °^W(H reasons, unreliable, and from their owń experi^^ri°^Us show that very thin films (< 10-⁴ cm.) of a numb^ts of liąuids, including water, are not only truły fl_u-f but have the same viscosities as they do in bulk Kingsbury finds ordinary viscous behaviour i_n lubricating films down to 6*io-⁵ cm. The whole ąuestion is still really unsettled. There is no doubt however, that certain long polar molecules do behave abnormally, and owe their lubricating properties to their abnormal behaviour.

Neale has studied the rotation of one brass tubę in another driven by an electric motor, the bearing being lubricated with various oils. He shows that even those oils which have short molecules, and are believed to be true fluids in normal flow, show a fali in viscosity at high rates of shear (as measured by the relationship between the armaturę current and I the speed). He ascribes the phenomenon to an I orientation of molecules, and ąuotes Kyropoulos as I having obtained similar results seven years earlier I (1930), his work having remained unnoticed. Apparatus for measuring the viscosity of surface (absorbed) films has been described by Harkins.¹

Ductility and tensile strength are of course, connected with viscosity; though if plasticity I and ductility are taken as being synonymous, it has been shown that the relationship must be a I very complex one. If two atoms approach each other, the resultant of the attractive and repulsive

¹ Ar. ezcellent article on oiliness appears in the Second Report on Viscosity and Plasticity of the Academy of Sciences, Amsterdam, 1938 Jvtde Prefacej. A psychological study of the poroepuon of oUuu~.> has been. madę by Sullivan and Cobbey iArtur f. Piychol., 1922, XXXIII., 12Sj.    ¹

pUCTILITY AND TENSILE STRENGTH 115

forces takes the form of a rise in the attractive force, until a certain optimum distance is reached. As the atoms approach still closer, the attractive force falls and reaches zero, after which a rapidly increasing repulsive force is experienced. The maximum attractive force corresponds to the tensile strength, and the corresponding extension to the ductility. If a tangent is drawn to the force/distance curve at the point of zero force, the intercept of this tangent on the (negative) force axis gives the elastic modulus.¹

The conception of shortness, when applied to doughs, is really a mixture of the strain and the load at which rupture occurs. Halton and Scott Blair show that it is connected with structural viscosity—(Stj/SS)^, but that the exact relation is not yet elear. The connection may probably be explained in this way: if a dough were a truły viscous fluid, a cylinder of dough would extend until it necked to a point, before rupturing. If it were a solid, it would break sharply at a certain stress (tensile strength). The morę “ solid ” and the less “ fluid ” the materiał (i.e., the morę sudden the breakdown of the resistance as the stress rises), the “ shorter ” it would be.

Another aspect of the ąuestion which may be considered is the distribution of yield-values within the materiał. A fibrous materiał, such as a “ short ” dough under strain, has markedly differentiated zones of strength and weakness.² As the stress rises, morę and morę of the local yield-values are

| Houwink:    " Elasticity, Plasticity and the Structure of

Matter ” p 23. The diagram given does not allow any appre-dable region over which Hook's law is obeyed ; but this may be

largely a ąuestion of the scalę of the drawmg

* See a recent article by Bndgman, J. Appl. Phys. (1938), IX.,