The distribution of cystine sulfur in the cortex of various animal fibers has been investigated by the staining of cross sections with mercury vapor. The receptivity to dyes of various portions of the cortex has been found to be inversely related to their sulfur content. After oxidation with peracetic acid or bromine, the cystine-rich portion of the cortex in bilateral fibers (the paracortex) swells more in aqueous solutions than the orthocortex. Fibers so oxidized swell greatly in dilute, neutral or alkaline, solutions of strong electrolytes; the swelling is repressed at higher salt concentrations.

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A new experimental approach is described for obtaining a quantitative measure of the wettability of filaments. This technique is based on the spontaneous capillary rise of liquid between two filament surfaces held in an adjustable v-shaped arrangement. No contact angle measurements are necessary, all information being obtained from simple length readings. The apparatus allows a filament sample to be tested in different liquids and conditioned without being removed from the test mounting. Results are expressed in terms of the free energy of wetting. Some dependency on filament spacing at the liquid source level has been observed, but none connected with the angle between them.

Devices for cutting notches of controlled depth in monofils and for manipulating notched specimens under the microscope are described. Chordal and circumferential notches have been cut in nylon moriofils, and the deformation of specimens in the neighborhood of the cracks, when tension (to rupture) is applied, has been examined microscopically. The behavior of the fiber material at the roots of notches is discussed and interpreted in terms of molecular structure and orientation. Breaks observed in flattened monofils that had been notched on an edge, in one case, and centrally punctured, in another, have been interpreted to be of near-brittle character. Analysis of the char acter of a fracture su: face at a notch suggests that two mechanisms are operative in the propagation of a notch or crack to rupture. Tensile tests made on monofils of various diameters, as well as on circumferentially notched specimens, reveal that breaking strength decreases as the diameter becomes larger. This is taken to imply that the inherent flaw size usually increases with an in crease in specimen volume. From a plot of the breaking loads of monofils having Chordal notches of varying depth, an estimate of the equivalent depth of the inherent flaws is obtained.