Friday, June 14, 2013

Scientific Exploits of Ben Franklin Part 3: Heat and Evaporative Cooling

Benjamin Franklin, ca 1783
One snowy but sunny morning in 1760, Ben Franklin carried out an experiment.  He took pieces of cloth of various colors, cut them to the same size, and laid the squares of fabric onto a layer of snow.  Then he waited.  When he came back, he noted that the darker pieces of fabric had sunk farther down into the snow, as more snow below them had melted.  Franklin discovered an important phenomenon that we take for granted today, namely that darker colors absorb more heat.  He took this simple observation further by proposing an important application for his discovery: wear lighter color clothing on sunny or hot days to stay cooler.  That seems like common sense knowledge to us today, but it wasn't back then.  The idea of color absorbing heat wasn't understood or ingrained in the habits of people in the 1700s.   
Franklin was very interested in the science of heat and cooling during his lifetime, and as a consequence he made several important observations about phenomena that we often take for granted today.  Read more after the jump....

Legend has it that this interest started on a hot July day in Philadelphia in the 1750's.  Ben was sitting in his room, sweating profusely despite a small breeze blowing in through his open windows.  He got up to change his shirt, and he noticed that the dry shirt he picked up felt much warmer than the wet shirt he had taken off.  He realized at that moment that the breeze actually felt cooler while he was wearing the wet shirt.  This intrigued him, and Ben conducted a series of subsequent experiments  in collaboration with a chemistry professor from Cambridge University in England, John Hadley.  Ben put various liquids onto the bulb end of a mercury thermometer and discovered that the evaporation of liquids causes heat loss.  When he took his thermometer and put liquid onto the bulb end, he watched the temperature drop as the liquid evaporated.  If he blew on the liquid to make it evaporate faster, the temperature decreased faster.   He carefully noted alcohol had greater a cooling effect than water, due to alcohol being more volatile than water, which means that alcohol evaporates faster than water.   He then tested ether, which is even more volatile than alcohol, and found ether to have an even faster cooling effect than alcohol. Ben discovered a correlation between evaporation rate of the liquids and cooling.  

In a letter from 1758, he described a thermometer experiment with ether as follows:

By dipping first the ball of the thermometer into the ether, it appeared that the ether was precisely of the same temperament with the thermometer, which stood then at 65; for it made no alteration in the height of the little column of mercury. But when the thermometer was taken out of the ether, and the ether with which the ball was wet, began to evaporate, the mercury sunk several degrees. The wetting was then repeated by a feather that had been dipped into the ether, when the mercury sunk still lower. We continued this operation, one of us wetting the ball, and another of the company blowing on it with the bellows, to quicken the evaporation, the mercury sinking all the time, till it came down to 7, which is 25 degrees below the freezing point, when we left off. — Soon after it passed the freezing point, a thin coat of ice began to cover the ball. Whether this was water collected and condensed by the coldness of the ball, from the moisture in the air, or from our breath; or whether the feather, when dipped into the ether, might not sometimes go through it, and bring up some of the water that was under it, I am not certain; perhaps all might contribute. The ice continued increasing till we ended the experiment, when it appeared near a quarter of an inch thick all over the ball, with a number of small spicula, pointing outwards. From this experiment one may see the possibility of freezing a man to death on a warm summer’s day.
As usual, Ben had a practical conclusion to draw from his experiments: fanning yourself on a hot day works by increasing air flow to increase sweat evaporation and cooling.  He recognized that it was the evaporation of sweat that kept you feeling cool.  This also led him to become an important proponent of drinking plenty of fluids on a hot day to avoid dehydration from sweat loss.  He also suggested that application of alcohol on bandages to an inflamed area would increase cooling and provide some pain relief.  Even today, this contribution to medicine is still important, as many over-the-counter remedies for sunburn or other mild burns contain alcohol, which helps cool the affected area through evaporation.

As interesting trait of Franklin's was his recognition that common sense, long-standing observations could have important implications for science, and conversely that science could explain common sense observations.  In this same letter from 1758, he noted that, while evaporative cooling was a "new" scientific observation by himself and his fellow scientists in Europe (back in those days scientists were often also called philosophers as well, hence his use of the term below), this idea was a common long-standing observation in other cultures.  He wrote:

It is but within these few years, that the European philosophers seem to have known this power in nature, of cooling bodies by evaporation. But in the east they have long been acquainted with it. A friend tells me, there is a passage in Bernier‘s travels through Indostan, written near one hundred years ago, that mentions it as a practice (in travelling over dry desarts in that hot climate) to carry water in flasks wrapt in wet woollen cloths, and hung on the shady side of the camel, or carriage, but in the free air; whereby, as the cloths gradually grow drier, the water contained in the flasks is made cool. 

In the 1700s, Europe and its universities were the center of scientific learning.  However, in contrast to many other scientists of his day, Ben believed that good ideas and good observations don't always need to come from someone with a European university degree or fancy scientific pedigree.  Franklin realized that ancient or less technologically advanced peoples, despite being less "scientifically" advanced in the minds of his fellow European scientists, nonetheless had a lot of wisdom to provide in their observations of natural phenomenon.  This most likely stemmed from Ben's own lack of a formal education, as we previously discussed.  Ben knew that observations and practices from cultures other than his own, ancient or otherwise, had an enormous potential to reveal important scientific principles.  Another example of Ben's respect for ancient ideas comes the observations that lead to his oil-on-water experiments, which we described previously.  He wrote against scientific snobbery in a letter from 1773: has been of late too much the Mode to slight the Learning of the Ancients. The Learned too, are apt to slight too much the Knowledge of the Vulgar. The cooling by Evaporation was long an Instance of the latter. This Art of smoothing the Waves with Oil, is an Instance of both.

While based on some ancient practices, Ben's scientific observations about sweat and evaporative cooling were still ahead of his time.  As humans, we need to keep our core (inside) body temperature within a narrow range, centered around 98.6° on the Fahrenheit (F) scale or 37° on the Celsius (C) scale.  More than a few degrees drift downward can cause hypothermia (hypo for too little and thermia for heat), which can occurs below 95° F (35° C).  Too high of a temperature is termed hyperthermia, which can be any temperature over about 99.5° F (37.5° C), which can be caused both by infection and fever as well as what we call heatstroke, which is basically caused by a failure of the body to be able to cool itself off fast enough.  In extreme cases, hyperthermia during heat stroke can result in temperatures over 104 ° F (40° C), a very serious and dangerous medical condition.   

Our bodies can regulate their temperatures in the face of a wide range of external temperatures.  However, this can be much harder during times of strenuous exercise or hard physical labor.  The cells in our bodies, particularly our muscles, break down adenosine trisphosphate (ATP), converting the chemical energy stored within the ATP into mechanical energy that is used to do work.  However, this process can be very inefficient, with as much as 70% of the energy lost as heat, particularly during times of high physical activity when cells are burning lots of ATP.  
During physical exercise, the heat generated in the muscles is can be transferred directly to the skin if the muscle is in close enough contact to the skin.  However, a lot of the heat loss from muscles occurs through the circulation of blood.  As blood circulates through the muscles, it absorbs heat and carries it through the body, distributing much of it to the skin.  Heat is lost through the skin through convection, which is the transfer of that heat to the surrounding air.  At normal room temperatures (70-75° F), your body is constantly losing heat through convection to maintain its proper temperature (98.6° F).  

Sir Isaac Newton (1642 - 1727)
Your surface body temperature is around 91° F (33° C), which is slightly cooler than your core body temperature (98.6° F or 37° C).   Dry convection only works when the surrounding air is cooler than ~90° F.  The larger the temperature difference between your body and the surrounding air, the faster convection will cool you off.  This is called Newton's Law of Cooling, attributed to Sir Isaac Newton.  However, when the internal heat generation happens too fast (as during strenuous exercise) or the surrounding temperatures are too high (as on a hot day), the simple convection process isn't enough to keep your body cool enough.  This is where sweat comes in.  When your body begins to sweat, surface heat is lost as the sweat evaporates.  The production of sweat itself does nothing to cool you down, as the sweat you produce is the same temperature as your body.  The heat is lost when the sweat evaporates as the water molecules go from liquid to gas (water vapor).  As water molecules escape from the liquid on the surface of your body to the air, they carry heat with them, which cools the remaining water molecules left in the liquid. This system works very efficiently, but is often complicated by humidity, which is a measure of the amount of water vapor already in the air.  As humidity increases, the air is more moist and can't absorb as much additional water, thus sweat evaporates more slowly, and there is a decrease in the ability of your body to lose heat by sweating.

Benjamin Franklin, ca. 1746
Losing fluid through sweating keeps us cool and prevents our bodies from overheating.  However, we obviously don't have an unlimited supply of fluid in our body.  The fluid lost during sweating must be replaced.  After exercising for as little as half an hour to an hour in excessive heat, it is common to lose as much as a liter (1 kg) of water.  This water needs to be replaced for our bodies to continue to function properly and not become dehydrated.  Dehydration can lead to extreme fatigue, and excessive dehydration (around 7-10% fluid loss) can lead to a drop in blood pressure, because there is less water around to create blood volume, which can lead to dizziness and fainting.  Even higher levels of dehydration (15-20%) can result in significantly increased body temperatures and organ failure, particularly kidney failure, which can result in death.   

A lot of the detailed knowledge that we now have about how the human body responds to dehydration comes from studies conducted on soldiers in the Nevada desert during World War II and published in 1947.  This studies very clearly defined the benefits of ingesting fluid during prolonged exercise.  However, the idea of the importance of fluid ingestion during exercise did not gain wide attention within the sports community until the late 1960s.  Before this time runners we actively encouraged not to drink during prolonged exercise.  What may seem like ludicrously simple common knowledge to us today was just not widely known or accepted even as little as 50 years ago.  We now know, though, that fluid replacement is very important and critical to athletic performance, and this has created an entire industry of sports drinks and other hydration equipment like Camelbak packs.  All of this has a partial root in Benjamin Franklin's experiments on evaporative cooling back in the 1700s. 

Text © 2013 TheMadScienceBlog

Sources and Further Reading
  • J.V. Hirschmann.  "Benjamin Franklin and Medicine."  Annals of Internal Medicine.  2005. 143:830-834.  (subscription or pay-per-view only)    
  • T.D. Noakes.  Lore of Running.  4th Ed.  Oxford University Press, South Africa.  2001.  
  • T.D. Noakes.  "Fluid replacement during exercise."  Exercise and Sports Science Reviews. 1993.  21:297-330. 
  • E.E. Adolph.  Physiology of Man in the Desert.  Interscience, New York.  1947. 
  • L.G.C.E. Pugh, J.L. Corbett, and R.H. Johnson.  "Rectal temperatures, weight losses, and sweat rates in marathon running.  Journal of Applied Physiology.  1967.  23:347-352. 
  • C.H. Wyndham and N.B. Strydom,  "The danger of inadequate water intake during marathon running."  South African Medical Journal.  1969.  43:893-896.
  • C.H. Wyndham, N.B. Strydom, A.J. Van Rensburg, A.J.S. Benade.  "Physiological requirements for world-class performances in endurance running.  South African Medical Journal.  1969.  43:9969.   
  • The quoted passages are from letters from B. Franklin to John Lining in 1758, which can be accessed here, and to William Brownrigg in 1773, which can be accessed here.
  • Images are pubic domain


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    David Klein

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