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The requirements for, and limitations of, a method of quantitative estimation of additives in a base material, measured by differences in density, are set forth. The specific gravity gradient column of Linderstrom-Lang is described and discussed in terms of its range, sensitivity, stability, and calibration. The bearing which these factors have on the method is illustrated by an example in which the concentration of a melamine resin, applied to a wool flannel for control of shrinkage, is determined.

Basic differences between tensile tests of single fibers versus similar fibers in arrays such as yarn, skeins, tow, and staple comhings keep the strength of arrays from exceed ing that of single fibers. The reverse holds true for strain. Moreover, experimental difficulties of achieving simultaneous straining among the fibers in an array further reduce the apparent strength and increase apparent strain, by amounts that depend on the extent of departure from simultaneity, i.e. the strain distribution in the array. Close equivalence of group tests of strength and maximum strain to single fiber measurements can be obtained even for the wide range of slackness observed in some tow, by recourse to a parameter that is not affected by departure from simultaneity of straining, namely, -the energy absorbed in rupture. Strength, maximum strain, and associated measures of variability can be calculated from energy via the tenacity-strain function for the fiber type. Good agreement with single fiber measurements has been obtained for acrylic fibers in the form of tow.

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