“quoted by” entries of the Othmer and Chen article which he says were “described erronerously,” is in excellent company. We had two feiver “errors” than the leader, Reid and SherLvood, but three more than Wilke and Lee. Quite correctly, Dr. Scott characterizes all sin as wickedness in his fulsome admonition, “This repetition of erroneous data, and data of lois accuracy of uncertain origin, emphasizes the wellk nown dangers of using third- or fourth-hand information in correlation work.” But we would like to submit that data or correlations going through the hands of three or four competent scientists in their attempts to correlate or determine mechanisms or mathematical models d o not necessarily lose thereby as w~ouldthird- or fourth-hand evidence examined in a court of law. As long as all the correlations are based on, or used with, the same experimental data, the respective value of the correlations may be compared. It’hat difference if some are a third- or fourth-hand attempt? O r would Dr. Scott consider only the first correlation made with any given data? We quite agree that only accurate data should be used-if available; and if there are ever sufficient, then no correlation for prediction will be necessary-one merely uses experimental results ! Also. kve agree that no data should be of “uncertain origin,” and we believe there is no question of the origin of the experimental data which we quoted. T h e geneology can always be traced. IVe confess that we didn’t point out a bar sinister here and there-some data reported by others were definitely calculated instead of measured ! Above: in item (e) of the 71 examples in our table, only 15
are listed by Scott as being calculated rather than measured, and he calls ”foul” on these. However, for our purpose a comparison of the existing correlations still can be made by eliminating these 15 systems. The results of such a comparison for the average absolute deviation d o not change relative positions greatly: Arnold, 15.71%; Gilliland, 15.20cc; HBS (estimated force constants), 9.57%; HBS (known force constants), 5.75%; Slattery, 9.2070; and our proposed correlation, 5.26%. As noted above. it is worthwhile to study the same data a third or fourth time; this may now be fifth hand! But the result is about the same! literature Cited
(1) Boyd, C. A , , Stein, N., Steingrimsson, V., Rumpel, W. F., J . Chem. Phys. 19, 548 (1951). (2) Emmert, R. E., Pigford, R. L., “Gas Absorption and Solvent Extraction,” in Perry’s “Chemical Engineers Handbook,” 4th ed., McGraw-Hill, New York, 1963. (3) Othmer, D. F., Chen, H. T.: Ind. Eng. Chem. Process Design Dcuelop. 1, 249 (1962). (4) Reid, R. C., Sherwood, T. K., “ T h e Properties of Gases and Liquids,” McGraw-Hill, New York, 1958. (5) Scott, D. S., Cox, K. E.? Can. J . Chem. Eng. 38, 201 (1960). (6) Wilke, C. E., Lee, C. Y., Ind. Eng. Chem. 47, 1253 (1955). .Ving Hring Chen Donald F. Othmer
Distinguished Professor Polytechnic Institute of Brooklyn 333 J a y St. Brooklyn, S.Y .
CORRESPONDENCE
CORRELATIONS FOR THE COEFFICIENTS OF GASEOUS DIFFUSION
SIR: M r . Scott calls attention to a situation which certainly needs correction. I suspect that many authors are in the habit of quoting published physical constants without checking on the original work and that this is done in fields other than molecular diffusion. Critical reviews going back to the experimental studies are much needed in this and other areas. Mr.
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FUNDAMENTALS
Scott would serve the profession well if he \could provide such a review, on which he has obviously made a good start. Thomnr K . Shemood
Departmelltof Chemicai MassachusettsInstitute of Technology Cambridgz, M a s s .
