and i t is important to use terms that do apply. Otherwise, there will he no end to confusion in the respects we have addressed earlier [J. Chem. Educ. 1986,63, 935-937; 1988, 65,393-3941. Note added in proof: Further comment has appeared (2,3). Literature Clted 1. This problem is given in the text Physical Chemislr). by Eggern. Gregory, Hslsey,and
Rabinnvitch, Why.1965. 2. Tykodi. R. J. J. C h e m E d u c . 1990.67.383-384. 3. Sehurnak~r,V.: Waser.J. J . C h e m Edue. 1990,67,3%.
Verner Schomaker California Institute of Technology Pasadena, CA 91 125
JGrg Waser University ol Washinglan Seattle, WA 98195
A Discrepancy ol Language To the Editor:
I enjoyed the article "Buoyancy Measurements for Teaching and Research" [1989, 66, 2671. The discrepancy in volumes was quite intriguing, and I spent two days on this paradox. The solution to the discrepancy is actually one of language. When one says that "the buoyant force is determined by the volume of the fluid displaced, the word displaced is not in the past tense, hut rather in the present tense, "being displaced". This becomes clear if we draw two figures. In the first figure
A
HA =H,,A+al A.AH = a1 = Vs = VL = volume of liquid being displaced A.AH = volume of liquid moved above original level, H, = VL = vs
In the second figure
False Clalms-Correclad To the Editor: Though I am not a chemist, I have found many of the pedagogical articles in this Journal quite stimulating, and in several of them I have found ideas applicable to my own teaching of mathematics. I must, however, take strong exception to a number of statements in the article "Viewing Stereo Drawines" " 11989.66.6641. . . . In particular, its authors make the following categoricalhut entirely unsupported-assertions: (1) "A stereo viewer is indispensable to visualize [stereo pairs] correctly." (2) Thoueh "Such stereo airs. . .are designed t o ~ r o v i d ethe correct three-dimensiohal appearance when viewed with a special device,. . .a mirror image of the true object is perceived with unaided eyes." In fact, both these claims are simply false. I t is true that the method of cross-eyed stereopsis "brings in an inherent error", hut the authors seem completely unaware that there is an alternative method of direct viewing-known as "relaxed stereopsis"-that does not suffer from this defect. In that method, the eyes are directed oarallel to one another. as thoueh eazine a t a distant ohiect., while they are focused close on the respective images of the stereo oair. I t takes some ~ r a c t i c eto master this skill. since the combination of p a r d e l viewing and close focu$ing is never required in ordinary situations: nevertheless. a ereat many people have learned to do it, A d the method icroutinelv taught in courses on aerial maD reading. Those of us w h o m e the relaxed method see exactly the same stereo image by direct viewing as we do with a separating card or magnifying device. Moreover, when the left and right images of a stereo pair are reversed (as they will be if viewed with the cross-eyed technique), the perceived stereo image will not be a mirror imaee of the true obiect. hut rather a ~seudoscooicone. in whiEh all front/back~rel&ionshipsare'reversed. i n the case of the double helix. the result will aooear as a reversal of chirality, hut in a &ore complex image, such as a scene of a tree in front of a building, the pseudoscopic image will hecome "very difficult and confusing" indeed, since the tree will appear to he pushed back through the building that is now in the foreground. The tryptych printing technique is of course useful: it enables either type of viewer to see a proper stereo image, and in cases like the helix, i t provides a compact format for viewing both chiral possibilities. In conclusion, it should he noted that some of the commercial software that has been developed for generating stereo molecular oairs "on screen" (for examole. . . ALCHEMY 11) has been de'signed to accommodateeither type of viewing, h i allowine- the user tochoose whether or not to reverse the left/ right images.
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John W. Dawson, Jr. Penn State York 1031 Edgecarnb Avenue York. PA 17403
HA = H d + a l A.AH = ol = V, = VL =volume of liquid being displaced A.AH - a 4 H = volume of liquid moved above original level, H, = vi
These two volumes are not the same, V LZ VL. In the first figure the upper part of the rod must he suhmerged farther to push more liquid above the original level, H,. This is not meant as a criticism, but a clarification. This article was useful. R. Thomas Myers Kent State University Kent, OH 44242
444
Journal of Chemical Education
Money Is the Answerl To the Editor:
I have read the recent litany of educational system illsand suggested cures ( 1 4 ) with a cynical glee. There is only one real cure for the problems: rnone)!Sure, teachers need to be treated like the professionals they are, given control of their professional lives, and given the &pp&t service8 they need to do the job, hut the key ingredient is a salary competitive with those in the orivate sector. "Eaual Dav for eoual work" is not just a femke demand, i t needsto-he ouis as well.
Though trite, "You get what you pay for" is also appropriate. My personal history may prove instructive. I teach a t a state college, the training ground for most future scientists and teachers. I have a PhD, six years of postdoctoral research, and 12 years of teaching, research, publications, and service as an assistant professor-all the "right" things for my profession. I recently was rewarded with the first of only two major promotions possible, that of associate professor rank. All the effort and sacrifice had finally "paid off', I was informed by an $80,00O/year administrator, as he awardedme a3.8% raise, whichmeans an extra $15 a week to my family. I am expectedto be grateful. What I am is insulted and demoralized. I have yet t o receive a salary equal to what a fresh PhD can expect to start with a local pharmaceutical firm. I have students who earn more waiting tables at local restaurants than I do. This situation, I find, is the norm. Far from being "alarmed" (6) a t the impending shortage of science and math teachers, I welcome it. I intend to do everything in my power to augment it by honestly and openly informing prospective teachers that society values the nurture of minds far less than the body. If they need to serve mankind, I'll suggest an MD program. People will pay dearly to unclog their nose, hut not to open what lies behind. And MD's have a better union, too. Terry L. Helser S t a t e University College Oneonta. NY 13820
Be Cautious of Hypothetlcal Treatments of Impossible Experiments To the Editor:
I-~should like to comment on examnle 2 of W. H. Seaton's paper on "Plosivity" [1989,66, ~ 1 3 7 j . Neelectine the certaintv that a mixture of nitric acid (1.04 mol),-acetic-anhydride (i.0 mol), and water (0.15 mol) will not exist as such for lone enoueh .. " to ~. ui ttinto a test avvaratus, all experience suggests that, though this mixture may not be "nlosive". the shravnel emanatine from auto-reaction will stili kill. here is evidence to suggest that it becomes detonable on standing ( I ) . Even if it be not ihork sensitive, I have seen runaway oxidations, which would have heen explosions at pilot-plant scale, with ahout a fifth of this proportion of nitric acid. Interestingly, if we assume that the water reacts, as it will, with anhdride. which means a lowered available ~ ~the ~-~~ - acetic potential energy, the mixture becomes more, not less, "plosive". On the other hand. if we assume acetvl nitrate formation, probably associatedwith increased detbnability of such mixtures on aeine. " -. the "~losivitv"declines. I would earnestly commend tdall your readers the study of texts covering experience of reality, such as L. Bretherick Handbook of Reactive Chemical Hazards, before they trust the lives of their students to hypothetical treatments of impossible experiments in that ideal universe inhabited by chemistry teachers but unknown to those who practice the art. ~
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Literature Clted
Literature Cited
I. Holden,C.Science 1989,244,1536-1537. 2. Judce, R. Chrm. Enp. Nexus 1989.67, 42. 3. On C o m p w 1989,9(1),2. 1. McKenns, B. OnCompus 1989.9111, 10-17. 5. Legowaki,J. J. J. Chrm. Educ 1989,66.535 6. Tifft. Time 1989.lSopt. 111,6b70.
I.BromT.A. Chrm. Brit. 1967.8, 504. Dingle L. E . e t al. Chrm. Rril. 1968.4. 136.
P. G. Urben 2 Upper Rosemary Hill Kenilworth.Warwickshire CV 8 2PA. England
S.
Aufbau Mnemonics To the Editor:
It would appear that a popular pastime of contributors to this Journal consists of devising new mnemonics for teaching electronic configurations, as well as debating the virtues and priority of various forms of these diagrams (1-5). I cannot resist participating in this sport by pointing out that, to date, nobody has cited the real originator of the idea, who was Sommerfeld (6). On a more serious note, however, the more concrete we make these mnemonics the more they are likely to be taken as a representation of the truth by students. Not only are there numerous cases of exceptions as many contributors have pointed out, but, more fundamentally, the assignment of electronic configurations to atoms represents an approximation that is stricrly inronsistent withquantum mechanics
(7). Perhaps we should also devise mnemonics to ingrain the latter fact into the minds of unsuspecting students. Literature Clted 3. 4. 6. 6. 7.
Strong. J . Cham. Edur. 1986.63. U. Adelhelm,M.:Hohn,E.G. J. Chem.Educ. 1988.65, 1114. ParsonsR. W. J. Chem.Educ. 1989.66.319. Sommerfeld.Mem. Roc. Monchesier Lit. Phil. Soe. 1925/26, 70,141 Scerri. E. R. J. Cham. Educ 1989.66.481.
Erlc R. Scerrl King's College, H.P.S. London University London SW3 6LX. U.K.
To the Editor;
Dubar and Calzia (1)conducted a series of experiments in which they subjected equilibrated mixtures of acetic anhydride. nitric acid. and water. within aluminum cylinders, to the impact imposed by a detonator that contained 5 g of mercurv fulminate. Thev vlotted their results on a ternary diagram to obtain a dome-shaped region that was based aeainst the acetic anhvdridelnitric acid side of the triangle. y h e area beneath this-dome defines all of the compositions that would he classified as plosive ifsuh.iected to this level of impact. In a second serie; of experiments, the impact was imposed by a detonator plus 10 g of "plastic" (believed to he about 10%more powerful than Tetryl). This series defined a second dome-shaped region that completely enclosed the first region. In a third series of experiments, the impact was imposed by a detonator plus 20 g of "plastic". This series defined a third dome-shaped region that completely enclosed the second region. No doubt, larger amounts of plastic would have enclosed even larger regions. However, the level of impact imposed by their third set of conditions can be regarded as very severe. As these results indicate, positive results can be accepted from almost any experimental test, but, when a negative result is obtained, there is always the possibility that more severe experimental conditions or the nresence of a (often unsus~ected)sensitizer will reverse the classification. 'one should always combine judgment with comvutational and experimental evidence to decide if a processbresents a signifiiant danger (2). Volume 68
Number 5
May 1991
445
