becomes. (Aspirin, incidentally, is much more soluble in alcohol than in water.) Physiologically this appears as a concentration dependent enhancement by alcohol of the penetration of the mucosal harrier by aspirin with consequent increase in damage. As Davenport elegantly puts it, "Cocktails and aspirin would therefore appear to he an insalutary combination." The coupling of acid-base equilihrium with the differences between polar and nonpolar solvents controls the behavior of substances in other parts of the body as well. For example molecular ammonia is soluhle in fats, hut ionic ammonium salts are not. Ammonia formed by kidney cells can pass through a lipid memhrane into the kidney tubules holding urine. Since the urine is acidic, once the ammonia is dissolved in this aqueous solution, it is converted to the ammonium ion and cannot get hack into the cells. In this case the purpose of the process is to partially neutralize acidic wastes produced by the body.
Physical Chemistry of the Drinking Duck Illustrating principles of phase equilibria and thermodynamics Suggestion by Professor Robert E. Wagner, Worcester Polytechnic Institute Millions of young people have had their sense of humor tickled by performances of a simple toy-the drinking duck. Give the duck an initial drink and he periodically dips down for more. His thirst is insatiable, yet he never drinks to excess and loses his dignity. The operation of the toy is puzzling to most people; in fact, the device is easily understood in terms of a few elementary physical chemical ~ r i n c i d e s . Tbe ev&oration of water, a spontaneous process if the partial pressure of water in the air is less than the equilihhum vapor pressure of water, serves as the driving force for the device. Increasing the ventilation makes the duck hob more ranidlv: . .. .olace a bell iar over him and he stonsyou have cut off his route of entropy production. Eva~orationof water cools the head, as thermal energy is needed to break the bonds hetween water molecul&, and increase their intermolecular potential energy. But how is the temperature difference between the top and bottom harnessed to produce mechanical motion? The duck contains methylene chloride-no air-just liquid and vapor. It is through the liquid-vapor equilihrium and a clever geometry in which hydrostatic equilihrium and mechanical equilibrium are mutually exclusive that the me-
chanical motion is produced. The duck has vapor in two places as shown in the diagram below. The vapor pressure of methylene chloride increases by about 15 torr for each "C increase in temperature, in the vicinity of room temperature. Hydrostatic equilihrium will he obtained if the height of the column, h, just balances the difference in pressure in the two volumes of vapor. From the density of the liquid we may calculate the height of the column which will produce hydrostatic equilibrium for a given pressure difference, and from the variation of vapor pressure with temperature we may calculate the temperature difference required to produce a given pressure difference, and hence a given column height. These are shown below for CHzClz, which has a density of 1.335 g cm-3. A temperature difference of less than a degree will push liquid up to the head and when this happens the system becomes mechanically unstable, the head dips, vapor leaks from the hase to the head, liquid runs back to the hase and the duck rights himself again-but now further displaced from hydrostatic equilihrium. And the operation continues as an oscillation between the mutually exclusive hydrostatic and mechanical equilibrium.
Volume 50, Number 3. March 1973
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