al use, then realistic testing should be made. Reliable opinions from a t least 8 different laboratories (references upon request) that have used the mercury detection apparatus described by Mr. Lawrence for monitoring work-room air, place severe doubt a s t o the efficacy of this particular device. In summary then, (1) it is recognized that diffusion certainly plays an important role in liquid systems (e.g., mercury-water); (2) if effective safety engineering is really a consideration, then relative diffusion coefficients are not sufficient to adequately evaluate potential occupational health problems; (3) if safety engineering is based on control of contingincies "before the fact" then further considerations in describing the system are necessary: (4) an industrial hygienist or occupational health consultant would probably first consider control measures based on good ventilation, a topic alluded to by both Lawrence and Hawkes. M. L. Sanders Oeeupational Health Service Naval Regional Medical center Bremerton, Washington
Ball Bearing Safety Hazard To the Editor: The ball bearing corrosion experiment cited by Slabaugh, [this Journal, 51, 218 (1974)], is potentially hazardous. We compressed a new hall bearing to 8 tons in a machine press and this hearing exploded within one or two seconds of immersion in acid, the fragments hitting the ceiling with some force at a height of twelve feet. D. A. Jenkins City of Leieester Polytechnic P.O. Box 143 Leicester, LEI 9BH, England
To the Editor: Professor Jenkins has raised an imnortant factor that u p to now had escaped us. Apparently, a hearing exceeding Rockwell hardness 60 had been successfully distorted without rupture, whereupon it exploded in the acid. It would be advisable to use bearings of less than Rockwell hardness 55 for this experiment. In any event, a high level of safety precaution should he exercised at all steps of the hall hearinn experiment. W. H. Slabaugh Oregon State University Corvallis, 97331
The Case of the Ever-Changing Partners To the Editor: It was a morning in late May of 75. "Holmes," remarked I, "surely you have noted the article in the Journal of Chemical Education by the professor who assigned everdifferent teams of pupils for practical work?" "You mean, Watson," replied he, "the Professor Marron who has never played tournament bridge? And that Journal is edited, if I mistake not, by a Professor Lippincott who has never played tournament bridge, either. Apart from that I know nothing of either professor." "Holmes!" cried I. "How could you know these things?" 820 / Journal of Chemical Education
"Elementary, my dear Watson. If either of them were a tournament bridge player, he could not have failed to notice that the good professor's problem is identical with that of conducting a tournament so that no team meets another team twice, and each team plays each board exactly once. So he need onlv have consulted his neiehborhood tournament director, or borrowed a copy of Beynon's book, to find all the ~rocedureshe needed. In fact." said Holmes as he warmed to his topic, "the movement "sed hy the good professor, in which half the group only meets the opposite half, is called the 'Mitchell Movement' after John T. Mitchell, who published i t in 1891 in "Du~licateWhist" bv McClure of Chicago. The more commonl; used movement, in whicK each team meets all others, is called the 'Howell Movement' after Edwin C. Howell, who worked i t out for all numbers of teams from 6 through 46. I do not remember exactly when, but it was about the same time, because he was cited by E. H. Moore in a memoir in the American Journal of Mathematics in 1896, volume 18, pages 290-
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"Remarkable, Holmes! But do you fault the professor's proposal?" "Not a t all, Watson, except that it has been in common use for the better part of a century. In fact the Mitchell Movement which is based on Graeco-Roman squares has a peculiar advantage: if the class is made up half of boys and half of girls, this movement can be set up so every team consists of exactly one boy and one girl. Setting that advantage to one side, the Howell is perhaps better; the teams are more widely mixed, there is a perfectly simple Howell movement for 12 (whereas there is none such Mitchell), and there exists a straightforward method of constructing a Howell for any number of players." J o h n H. Watson, M.D. Richard E. Powell
C/O
University of California Berkeley, California 94720
Tygon Stirring Bars To the Editor: I t should be pointed out that A. A. Himmel's [J. CHEM. EDUC., 52, 264 (1975)l Tygon magnetic stirring bars should not be used with organic solvents. Many red-faced researchers can attest to the fact that organic solvents will leach the phthalate ester plasticizer (e.g., di-2-ethylhexyl phthalate) from polyvinyl chloride (Tygon) tubing. For example, from a 1%-in. piece of ordinary Tygon tubing (R-3603; % in.) a t room temperature in contact for 1 hr with 30 ml each of methylene chloride, diethyl ether, benzene, acetone, 95% ethanol, hexane, or methanol there can he obtained 0.61, 0.59, 0.56, 0.66, 0.03, 0.33, and 0.04 g, respectively, of di-2-ethylhexyl phthalate (by nmr). Since even blood can leach phthalates from Tygon tubing (Jaeger, R. J., and Rubin, R. J., Science, 170, 460 (1970)), the Himmel magnetic stirring bar should probably not be used with aqueous solutions. And it is certainly not recommended for use with organic solvents. Gordon W. Gribble Philip D. Kntzenco DartmouthCollege Hanover, New Hampshire 03755
