|Benjamin Franklin, ca 1783|
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....
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.
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:
...it 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)|
|Benjamin Franklin, ca. 1746|
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