cil for legends and B pencil for thin lines, but this is purely a matter of artistic taste. One advantage of this technique is that it can easily be done in color, since ordinary office type red, green, or blue pencils come out beautifully. The Venus 200 series are fine, but other colored pencils work. When the drawing is done, lay it face upward on newspaper or paper towel and spray lightly with clear Krylon or anv, eouivalent acrvlic lacquer. After a few minutes the slide hecomes transparent and permanent. Mount in u standard Polaroid 314 X 4-in. mount After the slides are once made, they do not take easily to changes, but notations can in fact be made or color added with Staedtler "Mars Lumocolor" pencils, designed for overhead projectors, and obtainable in shops specializing in audio-visual supplies. Since the matte acetate is inexpensive and the Polaroid mounts can be reused forever, the cost of one of these slides is negligible. And an almost infinite number of them can be stored in an envelope. Credit should be given to the original inventor of this system. In the 1950's a company called "Spray-Vue Enterprises," of Elizabeth, N. J., marketed a similar product. It was made of 15-mil vinyl, which is thick enough to he self supporting. The product appears to have been a commercial failure, but the idea was good. However, the use of 8-mil acetate and Polaroid mounts, as described here, is far superior.
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Richard E. Powell University of California Berkeley. 94720 Practical 2
X
2 Slides
To the Editor: Recently, Sigred Lanoux describes [J. CHEM. EDUC., 50, 476 (1973)l a method for making convenient and inexpensive lantern slides. We have found that inexpensive 2 x 2 slides can be made in a shorter period of time a t less expense and with high professional quality. 2 X 2 slide film which has a surface for writing can be obtained as -~ -~ "U"-Film from Educational ~ r o d u c &Division of Hudson Photoeraohic Industries. Irvin&on-On-Hudson, New York . 10533. We have found that high quality slides can be prepared by typing on the film with carbon ribbon as well as hand drawing using a traditional pencil, colored fiber tip pens, or India ink. Letters such as those described by Lanoux will work well but require more effort and expense. The film can be erased with a gum eraser and used several times. Once the slide material has been prepared, the film is mounted in a plastic holder, available from most AV suppliers under such trade names as Kindermann. The slides can be prepared in 5-15 min depending on the graphic work required. Slides prepared by this method c a t approximately $0.05 per slide. Jimmy C. Stokes Stephen B. Walter Clayton Junior College Morrow. Georgia 30260 The Return of the Hummingbird To the Editor: In reviewing previous issues of THIS JOURNAL to find interesting and illustrative problems for the enjoyment and edification of undergraduate students, we found the 798
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following two passages that deserve further comment. In regard to the note of E. R. Gerlach (J. CHEM. EDUC., 46, 455 (1969)) and the subsequent letter of G. Nebhia (J. CHEM. EDUC., 46, 701 (1969)) on the efficiency of the Volkswagen versus the hummingbird, we would like to point out the following error. Hummingbirds do not in general use gasoline as a primary energy source, nor do Volkswagens normally use animal fat (as was assumed in G. Nebbia's calculations). Therefore, making this correction, the efficiencv of the humminabird is 1320 mi-elkcal. while the ~ o l k s w a ~ ecan n onl; manage 640 mi-gjkcal: Thus the humminabird - is 2.1 times as efficient as the Volkswagen. However, in light of the current energy crisis and the increased use of emission controls, we believe a recalculation of the data is in order. We make the following assumptions 1) The
primary energy source of the Volkswagen is gasoline (using the heat of combustion of n-octane. "Handbook of Chemistry and Physics," 51st Ed., Chemical Rubber Co., Cleveland, Ohio, (1971). 2 ) The standard Volkswagen is a 1973 Super Beetle Sedan with a curb weight of 1984 lb (TriangleVolkswagen, Durham, N. C.). 3) One standard passenger weighs 200 lh. The calculation proceeds as described by Gerlach, with the exceptions that the density of gasoline is 5.6 lb/gal ("Handbook of Chemistry and Physics," 51st Ed., Chemical Rubber Co. (1971)) and the gas mileage of the sedan is taken to be 25 milgal. The sedan with one passenger (the driver) has an efficiency of 855 mi. Using the revised humminehird calculations. the relative efficiencv of the hummmghird to ihe \'olkswagen is 1.25. To illustrate the arl\xntaae of n car~ool,we haw romputed the efficiency of the sedan wit6 a driver and three passengers (2784 lb total weight) assuming no significant loss in gas mileage. The result is 1090 mi-g/kcal. The relative efficiency of the hummingbird to the Volkswagen is in this case 1.21. The caroool is 1.28 times as efficient as the driver only. The suoremacv of the humminebird over the Volkswagen should ~ro;ide E. R. Gerlach with peace of mind.
L. P. Beaulieu D. E. Booth J. T. McCown W. R. Denan, Jr. University of North Carolina at Chapel Hill Chapel Hill. North Carolina 27514
