tion to models that no longer correspond to current theoretical thought. I t should seek to use science which is as invariant as oossible when formulatine- "ex~lanations". In addi. tion, whrn recourse ro modrl:, ii made, it ought to be honest enough toemohasi~ethe defects ola i~articularmodel and to exp& why chat model was chosen in spite of its failings. On the other hand, when it turns to a discussion of measured quantities, it should give a suitable critical explanation of the manner in which the data were obtained. In short, mastery of empirical fact goes hand in hand with critical thinking and with correlation and generalization. It is amusing to wonder what our role as teachers would he if we were to turn to an encyclopedia of descriptive chemistrv. -, devoid of d o t . Would we oreach from the text hut provide our ow; exegesis? Or m&t we stick to the written word? The latter course suggests we would he teachingread-
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Acknowledgement I would like to acknowledge the helpful suggestion of a reviewer. W. Danlel Goodman College of Great Falls @eat Fails, MT 59405
MOLDOT To the Editor: I was interested in reading about the program MOLDOT that produces space-filling diagrams of molecules [1986,63, 3261. However, I was disappointed that the figures omitted hydrogen atoms, which constitute much of the outer surface of organic molecules and thus determine much of their "visible" form. i.e.. their steric orooerties. I t is our ex~erience that many postgraduate stuienis learning crystal structure determination. if asked to eenerate mace-filline diaerams, tend to omit hydrogen atoms and &us to produce such "unrealistic" diagrams. I t is a paradox of X-ray structure determination that the hydrogen atoms, which contribute so little to the diffraction pattern, play a major role in determining the molecular packing in the crystal. Clearly, the omission of hydrogen atoms reduces computing time. Equally clearly, we should not encourage the misconception that a molecular model without hydrogens is realistic. ~~~
John L. Ragle University of MaSSaChUSenS Amherst, MA 01003
Llterature Cited 1. Rer1in.T.
J.Chem. Phys. 1951,19.208.
2. Hirschfeider. J. 0.: Curtis, C. F.;Bird, R. 9. Molocvlor Theory o/ Cases end Liquids:
Wiley: New York, 195kpp932-937. 8. 9amrai.A. S.: Deb, B. M, Re". Mod.Phys. 1981,53,95-126. 4 . Par.. R. G.Ann. Rsu. Phys. Chem. 1983,34.631-656. 5 . Deb, 9 . M. ( E d . ) . The Force Concept in Chemistry; V a n Nostrand-Reinhold: New York, 1981. 6. Bader. R. F.: Nguyen-Dang,T.T.:Tal. Y.Repta,Plog.Phys. 1381.44 893-946. 7. StewarLR. F. C h ~ m . P h y s . L a l l .1379.65.335-342.
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Peter G. Jones institlR fur anoraanische Chemie ~&mannstrasse 4 3400 Gdhingen Fed. Rep, of Germany
Cool But Not Quick To the Editor:
To the Editor:
I read Cohen's article, "Temperature, Cool but Quick" [1986,63,1038], photocopied it for the students of my physi-
The program MOLDOT draws whatever atoms are specified in the data file. Two of the four data files supplied with the MOLDOT disk include hydrogen atoms. (See structure of cubane, below, in which hydrogen atoms are included.)
cal chemistry class, and introduced Cohen's article for discussion. I focused the discussion hy offering a slightly altered interpretation of the information and by asking the studenu to rhoow. In thought experiments, we looked at the exnansion of eases. the distribution of velocitv curves, and a t &hen's ambLguity. To try to understand the ambiguity, we used a supersonic jet plane analogy. First, we agreed with Cohen that the cooling of a gas by expanding it from a region of high pressure into a region of low pressure is a routine procedure. Next, we agreed with Cohen's suggestion that the width of the Maxwell-Boltzmann distribution is identified with temperature. By the use of a jet plane and exhaust analogy, we noticed, however, that we could understand the source of the Cohen ambiguity. For instance, viewed from inside a jet plane, the exhaust would appear to be speeding away from the plane; from earth, however, the plane would appear to be moving away from its exhaust. T o measure directly the temperature of the plane exhaust, we decided, one would position a thermometer in the exhaust molecules. We decided to specify the frame of referenre tor the supersonic jet and molecules PIueriment as the tmmerarure meas~rinathr distribution of molecules. With the thermometer as the reference, we found that ambiguity about the distribution vanished for us in Cohen's jet-and-molecules thought experiment. In particular, we experienced ambiguity when we visualized the collection of molecules a t 10 K as speeding away from the stationary jet. However, because the laws of physics must be the same for all frames of reference, i t is equally valid, but unambiguous, to consider the jet source as moving away from the stationary thermometer and the collection of molecules at 10 K, in the manner of a jet plane speeding away from its exhaust. 746
Journal of Chemical Education
Omission of the hydrogen atoms in the figure used to illustrate my article describing MOLDOT does not greatly reduce computing time, which, as explained in the article, depends chiefly on the number of dots, rather than on the number of atoms. Steven Brumby me Australian Natlonsl University G.P.O. E m4 Canberra. ACT 2601. Australia
