parently it has not received the attention it deserves, and thus has not (at least yet) been examined in practice by many other leading workers. While I thought it worth notice, I could not recommend it for this reason. It would certainly be very helpful if interested parties would calculate moments by Estok's procedure as well as by conventional methods. I would like to take this opportunity to call attention to the very promising recent work of J. R. Weaver and R. W. Parry [Inorg. Chem., 5,703 (1966)l on calculation, of dipole moments. This is an examination, both theoretical and experimental, of certain procedures not following Debye's approach, and should be particularly interesting to those who wish to determine moments in pure liquids or polar solvents.
To the Editor: Dawber, Brown, and Reed in "Acid Catalyzed Hydrolysis of Sucrose [THIS JOURNAL, 43, 34 (1966)l focused attention on a reaction which has a teaching potential that is often overlooked. Generally sucrose inversion is used as a convenient reaction to introduce polarimetry or to illustrate firs& order kinetics. Dawber, Brown, and Reed pointed out how the reaction could be used to illustrate the Hammett acidity function. I suggest that all of these a3pects-and more--can be conveniently covered if the reaction is utilized in an "open end" type of investigation. I have found the following sequence to be both practical for the professor and satisfying for the students. Students are told that the phenomenon of inversion occurs, and they are asked to investigate the effect which changing the sucrose concentration (at constant pH) has on the rate of inversion. Each student (or pair of students) prepares a solution and observes the changing rotation for one hour. By plotting the ratio of the original sucrose concentration to the concentration remaining at time (t) vs. time, the class obtains a family of curves. These data serve as an excellent basis for the introduction of an application of statistics (Are the differences in the curves significant?) with the fringe benefit of a conclusion that surprises many. Tentative conclusions about the order of the reaction can also be proposed. A student may spot the exponential appearance of the curves. A plot of the log of the ratio of the coocentrations vs. time confirms the sound foundation for his suspicion. Further, because of the relationship that the slope of the line of this plot has to the reaction rate constant, the expression for the rate constant of first-order reactions can be derived directly without using differential equations. A11 investigation of the rate of sucrose inversion with changing pH is a reasonable followup to the previous work. Such an investigation yields, ultimately, the family of straight lines of differing slopes which confirm
the firseorder character of the reaction. With a little prodding, the students recognize that this also provides an insight into the hydrolysis nature of the reaction. This leads to a full discussion of the reaction inechanism. Thoughtful students notice that the slopcs of the lines do not appear to increase proportionately with changing pH. A plot of the reaction rate vs. acid molarity gives a curve such as Dawber, Brown, and Reed present (Fig. 4). The unexpected implicatioiis drawn from this curve prepares students for the discussion of the Hammett acidity function which terminates the investigation. The collection of the data and the discussions take five laboratory periods. The data obtained even by beginning students are very satisfactory, if reasonable care is taken. I shall be glad to furnish details of appropriate concentrations and specific techniques to interested readers. Obviously this particular sequence does not exhaust the instructional possibilities of the approach. However, whether this approach is used or not, the combination of the intrinsic simplicity of the reaction and the quantitation possible with a polarinieter merits more attention as a teaching tool.
To the Editor: Dr. Pode's critical reflections on the CBA and CHI311 Study programs [THISJOURNAL, 43, 98 (10(i(i)]werc most interesting. I was particularly happy that he did not pit one program against the other or align himself with those who would merge these studies into the summum bonum-the perfect high school chcmistry course. As a former member of the CHEM Study staff, and as one who is using the course materials for the fifth successive year, I admit to considerable bias ~vhiclr was born of much sweat but few tears. The impression that the CBA teacher's guide "leans more toward helping with teaching" while that for CHElI Study "indulges in more background discussion" may appeal to the CBA teacher hut I feel that CHEM Study teachers will demur. Of course, these discussions are helpful, but they represent only one (perhaps minor) pillar of support to the inexperienced teachcr. The emphasis in the CHEM Study (T.G.) is on intent and approach, development of ideas, the class schedule, lab hints, and the solutions to the exercises and problems. I wonder if the claim that CHEhl Study is guilty of "introducing ideas without adequate discussion" is shared by practicing teachers? I t has been my esperience that the most useful textbook is characterized by what it omits rather than by what it includes. Undoubtedly, more could have been done in the areas Dr. Pode describes, but how far do me go with the average high school youngster? Volume 43, Number 7 2, December 7 966
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I disagree with Dr. Pode's final punch-line, "In the best schools three chemistry courses should be available: CBA, CHEM Study, and an Advanced Placement course to cater to those who have been fired with enthusiasm by either of the others." It is not important that a school offer CHEM Study or CBA or even an advanced placement course. The definitive course has yet to be written; the specific track we take is not important. Also, in the light of our new courses in chemistry, we should take a fresh look a t Advanced Placement. Whether such work is still needed and whether the high schook are offering the equivalent of a first year college course (what is it?) are moot questions.
To the Editor: Mr. Saul Geffner's remarks about the Teacher's Guides illustrate rather clearly the competitive spirit in which the relative merits of CBA and CHEhl Study tend to be approached in the USA-which I deliberately tried to play down in my article. Naturally enough the CBA guide has references to further reading, whereas the CHEM Study guide helps with some specific classroom queries; hut I doubt that an impartial reader will disagree with my statement that the stress in the CBA T.G. is on helping with specific classroom situations, whereas the stress in the CHEMS T.G. is laid on giving the teacher a sense of the structure of chemistry at a more advanced level than the student could understand. Both these aims are valuable, but they are different. To the outsider it is the extraordinary divergence between CBA and CHEMS in the means they adopt to produce the same result-the enthusiastic freshman chemistthat is so startling. My punch line--like most punch lines-can be misinterpreted. By dwelling on the differences b e tween CBA and CHElLlS and then saying that every school should have both, I was pleading for variety, not uniformity. I am sure that Mr. Geffner does not believe that by labeling a chemistry course as CHEM Study, that course is necessarily closely defined. Courses with that label as taught in the private schools in the northeast are considerably different from those in San Antonio or in San Francisco. The function of curriculum experiments is to provide materials for the teacher to use; no two classes are identical, and the teacher-if he is more than just a public address system-will produce his own highly personal approach to either course.
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Finally, on the subject of Advanced Placement, I agree entirely that an attempt to duplicate some of the highly original first year teaching which is emerging in many colleges in the USA, without the advantages of highly qualified staff and expensive equipment, is asking for trouble. Oxford and Cambridge have suffered in the past from schoolmasters "skimming the cream" from the first and second year university work, with the result that such topics contain no e l e ment of intellectual surprise or excitement for the undergraduate. I believe that a year spent in applying the theoretical concepts already covered to further e l e ments and periodic groups, extending the organic chemistry to a deeper understanding of functional groups supported by laboratory work, relating theoretical chemistry to the study of raw materials and industrial processes, and finally attempting a small-scale project to learn how difficult research is, gives a prospective freshman valuable experience without blunting his appetite.
To the Editor: I n an article discussing molecular symmetry and point groups [M. ZELDIN,Tms JOURNAL, 43, 17 (1966)], the author makes the statement that the Schoenflies system is "restricted to point group symmetry" while the Hermann-Mauguin system is "applicable to both point and space group symmetries." While the s t a t e ment is technically correct, it seems that it could have been worded a little more clearly. As it stands, it leaves the false impression that the Schoenflies system cannot he used to designate space groups. The Schoenflies system has indeed been used to designate space groups, but it does so in terms of the point groups from which they are derived without indicating other symmetry elements which may have been introduced. I t is this limitation that led to the development and adoption of the Hermann-Mauguin notation. The Hermann-Mauguin notation indicates all the symmetry elements present in a point or space group while at the same time labeling it. Use of this system does not require one to study complicated charts such as Figure 7 in the article referred to. This symbolism was first introduced by C. Hermann [Z. Krist. (A) 68, 257 (1928)l and later simplified by Ch. Mauguin [Z. Krist. (A) 76, 542 (1931)l.
