Table 1.
2 1 = 3 -Az+i
(5)
A similar analysis indicates that Equation 6 of Wronski is also incorrect for distributions other than “monochromatic”-i.e., all fibers originally of th,: same length-and the correct form is: I1 $- (CV)ZIn =
3 - h 2
(6)
where CV is the coefficient of variation on a weight basis of the original distribution or CV = U J R . The technique might have some merit if the CV typical of the original distribution were known or could bse estimated. Thus the error entailed by his Equation 6 for the distribution in his Table I which has a CVof 33.3% is about ll‘x. Table I presents the experimentally “observed” values for the combined I/*- and 1-inch fibers in Wronski’s Table I, theoretical values calculated from the development above, and the theoretical values to be expected for this combination based on his “observed” values for the l / ~ -and 1-inch fibers measured separately as given in his Table I. Recognizing that in such a clamped sample the expectation of catching is twice as great for 1-inc:h as for I/z-inch fibers, a weighted average of Wronski‘s “observed” values for the separate 1-inch and ‘/Z-inch distributions permits computation of the “combined” values shown here in Table I. The degree of agreement with the true theoretical values is good, with the only significant departures at the longer lengths, as was observed for his 1-inch fibers. This would seem to indicate that a measurable amount of breakage has taken place as noted by Wronski. Also to be considered (Tallant et al., 1968) is the necessity of having a “s.harp” clamp to prevent slippage by
7/8
Cut Fiber Statistics
Length, Inch 3/8 4/8 518 618 718 W t . 7” Less than Indicated Length 13.2 33.2 55.1 67.1 84.0 95.8 2/8
8/8
Observeda 2 . 9 100.0 Theoreticalb 3 . 1 1 2 . 5 2 8 . 1 5 0 . 0 5 9 . 3 7 0 . 8 8 4 . 3 100.0 Combinede 3 . 3 1 1 . 3 3 1 . 7 5 0 . 1 6 0 . 0 7 4 . 7 9 0 . 8 100.0 b Calculated a From Table I of Wronski for 7 - and ’/%-inchfibers. from Equation 5. c Calculated from data in Table I of Wronski for measurements of 7- and ’/%-inchfiberstested separately.
fibers extending only a short distance into the clamps. The inch or less. data suggest slippage of fibers clamped Wronski’s decomposition or differentiation in the latter parts of his manuscript is interesting but was not fully investigated in view of the fundamental disagreement of our basic equations. literature Cited
Pittman, R. A., Tallant, J. D., “Mathematical Models and Measurement of Fiber Hooks,” Textile Res. J., in press, 1968. Tallant, J. D., Pittman, R. A,, Textile Res. J . 38, 149 (1968). Tallant, J. D., Pittman, R. A., Patureau, M. A., Textile Bull. 94, 30 (1968).
Tallant, J. D., Pittman, R. A., Schultz, E. F., Jr., Textile Res. J. 36, 729 (1966).
Wronski, J., IND.ENG.CHEM.FUNDAMENTALS 6 , 595 (1967). J o h n D. Tallant Biometrical Services U . S. Department of Agriculture N e w Orleans, L a . 7 0 7 1 9 Robert A . Pittman Southern Regional Research Laboratory h’ew Orleans, L a . 7 0 7 7 9
GASEOUS DIFFUSION AND FLOW IN COMMERCIAL CATALYSTS AT PRESSURE LEVELS ABOVE ATMOSPHERIC SIR: The question at issue is whether or not surface diffusion of helium makes a1 significant contribution to the total flux of helium through a porous solid of high surface area a t ambient temperature and atmospheric pressure. Hwang and Kammermeyer have concluded that it does, on the basis of their observation that the product of the flux and the square root of the absolute temperature varied in their experiments with unfused porous Vycor by about 10% over the range of 77’ to 587’ K . If surface diffusion of helium at ambient temperature is indeed significant, this conclusion is important, for it affects the common use of helium as a reference gas for diffusion measurements in porous materials, as in our studies. Any scientific investigator who reports a new phenomenon, particularly when it has not been observed previously by others working under similar conditions, has an obligation to the scientific and technical community to discuss his experimrntal methods, observations, and interpretation with particular care so as to assure the community that he has considered, as far as practicable, possible spurious effects and other conceivable interpretations of his results. References cited (Hwang, 1965 ; Hwang and Kammermeyer, 1966a) had insufficient information to convince us thalr the effect was real. The additional
information in the above letter is useful and perhaps the surface diffusion of helium will eventually be proved to be significant under the conditions of interest, but the evidence is still questionable. Data on deuterium, methane, ethane, and propane, interesting as they are, are not data on helium. Surface diffusion of helium being significant at cryogenic temperatures does not mean that it is necessarily significant a t ambient temperature. The correlation of the parameter of surface diffusion with the critical properties and other molecular parameters of gases in general (Hwang, 1968) is described by Hwang (1968) as “semi-empirical.” No new experimental data are presented and the comparison of theory and previous experimental data is for krypton, ethane, propane, and butane. I n the later paper, Hwang and Kammermeyer (1966b) reported another set of data for helium over the same temperature range, presumably obtained with the same apparatus, which essentially showed the reproducibility of their earlier measurements and the same effect of temperature. They further state, “The same result was reported by Ash et al. (1963) for the permeability of helium through a microporous carbon. These authors found a decrease of as much VOL. 8
NO. 1
FEBRUARY 1969
175
as 15% with decreasing temperature." But Ash et al. published no such statement and Hwang and Kammermeyer have apparently misread Figure 1 of their paper. Ash et al. observed the permeability of S02, COz, N2, and Ar through a porous carbon plug a t low temperatures in an investigation which occupied two to three years. From time to time the helium permeability was measured to check whether stresses as produced by temperature changes and capillary condensation caused a change in permeability of the plug. Each such check would presumably be made under the same conditions. The helium permeability indeed drifted downward about 15% over the course of the experiments. The sequence of experimentation happened to be from SO2 a t -10' to -33.5' C., to COzand N z at -83' C., to Nz and Ar a t -196' and -183' C., but the plug was outgassed at 100' C. between runs. Figure 1 presents a historical account of the variation of plug permeability to helium with sequence of experimentation. Hwang and Kammermeyer appear to have interpreted it as the effect of temperature on helium permeability.
The structure of porous Vycor differs from that of most other porous substances of interest as it is prepared by a leaching process. In current studies at M.I.T., Koch (1968) is determining whether the effect of temperature on helium permeabilty, as reported by Hwang and Kammermeyer, can be observed in a porous disk prepared by compressing silica powder. literature Cited
Ash, R., Barrer, R. M., Pope, C. G., Proc. Roy. Soc. (London)
A271, l(1963). Hwang, S-T., submitted to A. I. Ch. E. J . (1968). Hwang, S-T., Ph.D. thesis, University of Iowa, 1965. Hwang, S-T., Kammermeyer, K., Can, J . Chem. Eng. 44, 82 (1966a). Hwang, S-T., Kammermeyer, K., Separation Sci. 1 (5), 629 (1966b). Koch, W. I., Massachusetts Institute of Technology, Cambridge, Mass., unpublished data, 1968. Charles N. Satterjield Massachusetts Institute of Technology Cambridge, M a s s .
INTERFACIAL AND ELECTRICAL EFFECTS ON THERMAL CONDUCTIVITY OF NEMATIC LIQUID CRYSTALS SIR: I t has come to the authors' attention that the specific heat of nematic p-azoxyanisole has previously been reported as 0.53 (Kreutzer, 1938)) compared to the value of 0.35 =!= 0.07 (Picot and Fredrickson, 1968). If correct, this value would displace the solid (theoretical) line of Figure 5 (Picot and Fredrickson, 1968, p. 87) downward by a constant value of 0.05' C. a t times greater than 0.025 msec. (and less at shorter times). This would have no effect on the conclusions arrived at by Picot and Fredrickson.
176
I h E C FUNDAMENTALS
J. J . C. Picot University of N e w Brunswick Fredericton, N.B., Canada
A . G. Fredrickson University of Minnesota Minneapolis, M i n n . Literature Cited
Kreutzer, C., Ann. Physik 33 (5), 192 (1938). Picot, J. J. C., Fredrickson, A. G., IND. ENG. CHEM.FUNDAMENTALS 7, 84 (1968).
