704 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Fig•re 4. 0.6 --•_ 443 - I 2 :5 4 5 6 TIME, MIN. Differential longitudinal swelling of two hairs in the same depilatory preparation arm. Very small changes in are may be discerned by this procedure. The result of this experiment was in agreement with the first. No rota- tion was detected. In contrast to the determination of cross-sectional swelling, the measurement of maximum rate of longitudinal swelling is quite straight- forward, independent of hair orientation, and practically independent of surface damage. The independent nature of the maximum rate of ex- tension is readily apparent if one considers its time of occurrence to be a functiou of the summation of the individual rates of extension along the entire hair length. If a small section of hair is damaged such that the time at which its maximu• rate of extension occurs differs appreciably from that which would be expected of the hair as a whole, its extension, being small by comparison to the total extension, will not affect a change iu T•,•. This may be seen in Fig. 4, where two hairs are placed in thio- glycolate solutions of identical concentration and pH. Although the plots are quite dissimilar (T/(443) = 4.0 minutes, T/(444) : 4.5 minutes) when T• is normalized to T•, the values obtained are equal (Tt(44a) = 4.0 minutes, and T/(444) : 4.2 minutes). These values approach the maximum precision attainable when T is small since readings are normally taken every 15 seconds. Sections of several hairs were abraded with wooden spatulas, and their T• values compared with the corresponding unabraded section. No appreciable change in T• was discernible.

QUANTITATIVE STUDY OF DEPILATORIES UI'ON IiAiR 2.6 2.4 2.2- 2.0- 1.8- 1.6- 1.2 0 4 6 8 I0 12 14 16 18 20 TI ME, MIN. Figure 5. Longitudinal swelling of hair in a depilatory si)lution The measurement of T•. offers one other distinct advantage over the measurement of TD. Although cross-sectional swelling occurs when hair is placed in solutions which are not capable of depilation (e.g., distilled water), longitudinal swelling does not. The plotting of differential curves is often cumberso•ne and time- consuming. An alternate method has therefore been adopted (viz., plot- ting hair lengths vs. time). This procedure produces a sigmoid curve (Fig. 5). If the segments of the curve comprising both sides of the first knee are considered linear, the intersection of these two lines is an excel- lent approximation of (T• q- T•)/2 (Figs. 2 and 5). This average •' is also inversely proportional to reactant activity. The approximation may be easily verified by comparison of Fig. 2 with Fig. ,5, both having been derived from the same determination. (T,• q- T•)/2, when ob- tained from Fig. 2, is 8.5 minutes by extrapolation in Fig. 5, it is 8.,5 min- utes. This approximation should not be used interchangeably with either Ta or T• to be meaningful it must be normalized in the same nmn- her as those functions. The normalized average approximation J'•./o has been utilized successfully in the evaluation of preparations produced in this laboratory. It is comparable to the time required to remove human axillary hair completely and is equal to approximately three times the time necessary for complete depilation of belly fur on test rab- bit. When evaluating opaque depilatories, the procedure may be modified by having the 1-g weight in the form of a thin steel rod. The sufficiently long rod is attached to the bottom of the hair in a manlmr such