Commentary pubs.acs.org/jchemeduc
Galilean Thermometer Not So Galilean Peter Loyson* Department of Chemistry, Nelson Mandela Metropolitan University, Port Elizabeth, 6031, South Africa ABSTRACT: A Galilean thermometer is a device for measuring the temperature of a liquid based on the density variation with temperature. It is named after Galileo Galilei, the famous Italian physicist, who apparently invented it. This article examines whether he did invent this Galilean thermometer and shows that he invented another type of thermometer, called a thermoscope or air thermometer, which is something completely different. The credit for developing the “Galilean thermometer” must go to the Accademia del Cimento, a research organization active in Florence from 1657 to 1667 under the leadership of Ferdinand II, Grand Duke of Tuscany. It is suggested that the “Galilean thermometer” should instead be called a “Florentine thermometer” after the place where it was developed. KEYWORDS: General Public, Continuing Education, History/Philosophy, Misconceptions/Discrepant Events, Liquids, Physical Properties
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floater or be resting at the bottom if their temperature tags have a lower temperature. The correct temperature of the liquid in the cylinder is indicated by the temperature on the suspended floater. If there is no suspended floater and some of the floaters are either floating on top or lying at the bottom, the temperature is bounded between that of the upper and lower floaters. The principle of operation is based on the temperaturedependence of the density of the liquid in the cylinder. Generally, a liquid to be used as a suspension liquid in the Galilean thermometer should have a density that increases with decreasing temperature. Each of the floaters has a total density that corresponds to the density of the suspension liquid at the particular, indicated temperature. So, when the temperature of the liquid is such that its density corresponds to the density of a particular floater, that floater will rise and be suspended in the liquid, thereby indicating the correct temperature. The other floaters will either have a higher density and lie at the bottom or a lower density and float on top of the liquid. A recent article in this Journal2 covers the construction of a Galilean thermometer using common chemistry glassware and is an excellent first laboratory in physical chemistry, which generates student interest, demonstrates the temperature effect of water on density, and emphasizes the importance of accuracy. Ideally, suspension liquids should have a density that changes greatly with temperature, so that they will be more sensitive to changes in temperature. For example it can be shown3 that within the temperature range 15−30 °C the density of ethanol changes 3.7 times as much as that for water and alcohol should therefore be preferred to water as the thermometric liquid. Suspension liquids used currently have even a better density− temperature dependence.
Galilean thermometer (Figure 1),1 named after the famous Italian physicist Galileo Galilei (1564−1642), can
Figure 1. Galilean thermometer.
be bought from various companies making scientific instruments. It consists of a sealed glass cylinder containing a clear liquid and suspended in this liquid are a number of small glass spheres, called floaters, each containing some different colored liquid. Each floater carries its own temperature tag, usually in the form of a little brass disk with the particular temperature indicated at which the floater will be suspended in the liquid. The other floaters will either be floating on top of the liquid if their tags have a temperature higher than that of the suspended © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: July 27, 2012 1095
dx.doi.org/10.1021/ed200793g | J. Chem. Educ. 2012, 89, 1095−1096
Journal of Chemical Education
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Commentary
DISCUSSION The system is a very visual way of temperature measurement, but did Galileo really discover this type of thermometer? In the period 1602−1606, while he was professor at the University of Padua, he made a thermoscope, 4 also called an air thermometer, for measuring relative changes in temperature. He took a “glass bulb with a long slender stem and as narrow as a straw; having well heated the bulb with his hands, he inserted its mouth in a vessel, containing some water, and, withdrawing the heat of his hand from the bulb, instantly the water rose in the neck more than a palm above its level in the vessel. It is thus that he constructed an instrument for measuring the degrees of heat and cold”, according to a letter written by Castelli, one of his pupils.4,5 The different degrees of temperature would then be indicated by the expansion and contraction of the air that remained in the bulb, so that the scale would be the reverse of modern thermometers, as the water would stand highest in the coldest weather. About 1611−1612 Galileo substituted spirit of wine for water and later still the Grand Duke Ferdinand II of Florence, a former pupil of Galileo, used colored spirit of wine and reduced the dimension of the tube.5 However, as long as the orifice of the tube remained open, this instrument could not be an efficient thermometer, as the expansive and contractive effect of the heat and cold could not be distinguished from the effects of varying atmospheric pressure. It was actually a barometer as well as a thermometer. It was not until 1653 that the practice of hermetically closing the orifice, after exhausting the air, while the contained liquid was in an expanded state, was introduced by Leopoldo de Medici, brother of Ferdinand II, thus depriving the instrument of its barometric character and making it a true thermometer.6 It is quite obvious that the thermoscope or air thermometer, discovered by Galileo, and its further developments and refinements has nothing to do with the Galilean thermometer, which was not invented by Galileo. Credit for this needs to go to the Florentine Accademia del Cimento (1657−1667), founded by the Grand Duke Ferdinand II and his brother Leopoldo, with its cautious motto, “Probando e Reprobando” (testing and retesting).6 This was a scientific group of academics and technicians and was the first organization founded for the sole purpose of making scientific experiments. During the 10 year existence they invented numerous forms of thermometers, hygrometers, hydrometers, and carried out investigations into a multitude of scientific topics.7 A thermometer, developed by the Grand Duke Ferdinand made use of glass balls floating in spirit of wine. The construction of such a thermometer is described fully in a document summarizing the experiments performed by the Accademia, dated 1667, and translated by Middleton.7 Six hollow glass-blown balls of slightly different densities are used. In ice water the balls should all float, and as the water heats up, each sinks as a certain temperature is reached. The Museo Galileo − Institute and Museum of the History of Science in Florence, Italy, has on display8 various forms of “Galilean thermometers” invented and designed by the Grand Duke Ferdinand II and his brother Leopoldo and co-workers, dating to the 1660s, which is after Galileo had died. One is a set of five phials or narrow tubes containing alcohol (spirit of wine), in which small glass spheres of different densities are immersed. A rise in temperature causes an increase in the volume of the liquid, which is reflected in the movement of the small spheres (first the less dense, then the more dense). This
thermometer has one sphere per tube. Another one has a number of spheres in a tube filled with alcohol. Another “Galilean thermometer” consists of six glass tubes, clustered on a column, each tube containing a small glass sphere of different density. These Galilean thermometers were made by skillful technicians from the Accademia: Francesco Folli da Poppi, Vincenzo Viviani, and Benedetto Castelli are some of the instrument makers who worked together with the Grand Duke Ferdinand II. However, the origin of the idea of using such a thermometer may well lie with Galileo as there is a passage in his book “Dialogues Concerning Two New Sciences” published in 1638 in which he describes a ball of wax, weighted to be in equilibrium with water, rise and fall when immersed in waters of different specific gravity and “so exact is this experiment that the addition of two grains of salt to six pounds of water is sufficient to cause the ball to rise to the surface from the bottom.”9 He carries on by saying that this notable difference in specific gravity can be produced not only by solutions of heavier substance, but also by merely heating or cooling.
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CONCLUSION The modern Galilean thermometer, supplied by various scientific instrument makers, was not invented by Galileo, but was designed and developed by a spirited group of academics and technicians belonging to the Accademia del Cimento under the leadership of the Grand Duke Ferdinand II in Florence during the period 1657−1667. Galileo did invent the thermoscope, also called an air thermometer, which is something completely different. A more appropriate name for the “Galilean thermometer” would be a “Florentine thermometer” as all the development work took place in Florence.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Galilean thermometer. Wikipedia: The Free Encyclopedia. http://en.wikipedia.org/wiki/Galileo_thermometer (accessed Jun 2012). (2) Padgett, C. W.; Padgett, L. W.; Priest, M. A. J. Chem. Educ. 2011, 88, 983−985. (3) CRC Handbook of Chemistry and Physics, 84 ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2003−2004; pp 15−33. (4) Middleton, W.E. K. A History of the Thermometer and Its Use in Meteorology; The John Hopkins Press: Baltimore, MD, 1966, pp 3−14. (5) Fahie, J. J. Galileo His Life and Works; John Murray: London, U.K., 1902. (6) Seeger, R. J. Galileo Galilei, His Life and Works; Pergamon Press: Oxford, U.K., 1966; p 11. (7) Middleton, W.E. K. The Experimenters: A Study of the Accademia del Cimento; The Johns Hopkins Press: Baltimore, MD, 1971. (8) Museo Galileo Home Page. www.museogalileo.it/en/visit.html (accessed June 2012) (9) Galileo Galilei Dialogues Concerning Two New Sciences; Hawking, S. , Ed.; Running Press: Philadelphia, PA, 2002; pp 53−54.
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