application~ and analogie~ Fire Walking, Temperature, and Heat Middle Georgia Collage Cochran. GA 31014
Warning: Although there islittle risk of personal injury far those who have been properly trained, students should be discourwed . from trving . .fire walking without professional instruction, For thousands of vears. ~ e o ~have l e marveled at and been puzzled by the abilcy of i r e walkers to walk across beds of glowing coals without avunrent harm. Historicallv, from an kmeriian science perspective, fire walking has been a foreign activity, performed in faraway lands such as India, Malaya, Japan, China, Tahiti, and the Fiji Islands. Many scientists, having never observed fire walking first hand, simplistically explained it away with assumptions such as "fire walkers have toughened and calloused feet." But many questions concerning this activity remain unanswered. Even as late a reference as the 1984 "New Encyclopedia Britaunica" concludes with the words. "IFire walking1 . . . has not been completely explained"1 TO&, cnw 30.&h Americans without thc henrfits of toughened and calloused feet encam .. in fire w a l k i ~ gHow . ~ thenao they do it? In general chemistry classes, teachers explain to students that there is a difference between the amount of heat objects contain (as measured by calories or joules) and the temperature of objects (as measured by degrees centigrade o r Kelvin). For example, sparks given off by a cigarette lighter are
' "The New Encyclopedia Britannica, Micropaedia"; 1984, 4. 152. McCouTt, R. Science 85 1985, 6 [March], 84.
976
Journal of Chemical Education
edited by: RON
DELORENZO
Middle Georgia College Cochran. Georgia 31014
very hot (about 1000 " C ) ,but when sparks from a cigarette lighter touch one's hands (or feet). one does not usuallv detect any warmth. Likewise, there is a difference between the amount of heat that glowing thousand-degree coals contain and their tempera&re. he dark footp;ints that momentarily form after fire walkers step on previously red coals indicate that the coals cool quickly. This cooling results in part by an ash compression and oxygen depletion at the glow front and by the heat flow from coals to feet. The specific heat of feet (which have a high water content) is much ereater than the snecific heat of coals.2 If red-hot coals were replaced by another red-hot substance with a larger specific heat. such as aluminum. the fire walkers would have roblems! A second factor to be considered is the conductivity of coal. Coal and coal ash are insulators. Once the surface of glowing coals is cooled by human feet, the heat transfer from the internally hot coal to the cooler coal surface is relatively slow. Again, if coals were replaced by a better heat conductor such as conner or aluminum. fire walkers would have additional problems! A third factor to be considered is the Leidenfrost effect. which is used to explain why water droplets spend such an unex~ectedlvlone amount of time on hot griddles before vapo;iziug. The bbttoms of water droplets i n hot griddles s float on insulatva~orizeverv auicklv, allow in^ the d r o ~ to ing layers oivipor. ~ a ~ i c i a n s u thid s e principle when they dip their wet fingers into molten lead.2 Fire walkers also dip their feet in ice water beforehand, or stand on damp grass at one end of the coal bed before their walk. The Leidenfrost effect may offer fire walkers the same protection received by magicians. Additional factors to be considered include: (1)coal beds
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are only 10 feet long, (2) coal beds are traversed rapidly with only two quick steps, and (3) embers are spread thinly.' When presented in first-year chemistry lectures, this author has found that student interest is piqued, student attention is elevated, and student comprehension of the important distinction between temperature and heat is improved. Addltlonal Readlng "Firewalking: Realiw or Illusion?", The Skeptical Inquirer 1985,IO.36 "An Investigation of Firowalking,l( The Skeglicai Inquirer 1985.10.23.
The Mystery of Excessive Perspiration While Jogging Ron DeLorenzo Middle Georgia College Cochran, GA 31014 Student interest can he heightened by a slight modification of the educational adage, (I) tell students what you are going to say, (2) say it, then (3) tell students what you said. The modification is (1) oresent an a~olicationof current interest as a myste&,and indicate ihat the forthcoming material is needed to solve that mvsterv. (2) teach thenecessary chemistry, then (3) solve the-mystery. Since over 50 million Americans jog or run,' typically over one half of all college students are very interested in learning more about physical fitness. Taking advantage of this interest, a discussion of colligative properties might begin by posing the following mystery: While jogging or running several miles, perspiration may be produced in such large quantities that it flows down the body and drins onto the e r o ~ n dSince . ~ one nurnose of oersoiration is to prddnce a cooll'ng effect by evapoiation, why'does the human
hudy nor produer just ~noughprrspiration to keep rhr skin aurfare muirr? The production of perspiration requires relativrig large amounts of energy, yet wasted effort is uncharacteristic of nature. In addition, excessive perspiration can lead to dehydration and death. What is the advantage .to the human body of producing more perspiration than wo"ld be needed to keep the skin surface just moist? After the topic of colligative properties has been developed, the original mystery is once again confronted. Students should he told that the following explanation is only one of many. (Keep in mind that the hody is simply trying to cool itself, and i t probably would not produce enough sweat that it would flow down the hody if one were jogging in a desert.) Perspiration contains solutes. If the human hody were to produce just enough perspiration to keep its skin surface moist, the solute concentration of the residual sweat would increase as the perspiration evaporates. As the solute concentration increases, the vapor pressure of this remaining concentrated solute solution decreases. This in turn decreases the evaporation rate and consequently the cooling rate.3 Therefore, it appears that one possible advantage of excessive sweating may he that excess sweat rinses away concentrated solute solution from the skin surface and thereby increases evaporation and cooling rates. Conrad. ~~.C. C. Amer. J. Saorts Msd 1981., 9~.199 'The extent of perspiration depends on several factorssuch as the amoient humldlty, the temperature, and the runner's condition. The salt concentration in trained runner's sweat is relatively low. Assuming this concentration to be negligible, then the vapor pressure of perspiration at 38 OC isabout 50 mm Hg. If the solute of a saturated solution of perwiration is 100% NaCI, and the DersDiration is then I is allowed to form a saturated solution (a saturated N ~ Csolution about 6.3 m with 12.7 moles of ions per kilogram of solvent), the perspiration would have its vapor pressure decreased by about 20%. ~~
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Volume 63 Number 11 November 1966
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