Inventing Temperature Read online

  1.

  Common boiling: numerous bubbles of vapor (probably mixed with air) rise up through the surface at a steady rate. This kind of boiling can happen at different rates or "degrees" of vigorousness, depending on the power of the heat source. The temperature is reasonably stable, though possibly somewhat variable according to the rate of boiling.

  2.

  Hissing (sifflement in De Luc's French): numerous bubbles of vapor rise partway through the body of water, but they are condensed back into the liquid state before they reach the surface. This happens when the middle or upper layers of the water are cooler than the bottom layers. The resulting noise just before full boiling sets in is a familiar one to serious tea-drinkers, once known as the "singing" of the kettle.

  3.

  Bumping (soubresaut in French; both later terminology): large isolated bubbles of vapor rise occasionally; the bubbles may come only one at a time or severally in an irregular pattern. The temperature is unstable, dropping when the large bubbles are produced and rising again while no bubbles form. There is often a loud noise.

  5.

  Explosion: a large portion of the body of water suddenly erupts into vapor with a bang, throwing off any remaining liquid violently. This may be regarded as an extreme case of bumping.

  Fast evaporation only: no bubbles are formed, but a good deal of vapor and heat escape steadily through the open surface of the water. The temperature may be stable or unstable depending on the particular circumstance. This phenomenon happens routinely below the normal boiling point, but it also happens in superheated water; in the latter case, it may be a stage within the process of bumpy or explosive boiling.

  6.

  Bubbling (bouillonement in De Luc's French): although this has the appearance of boiling, it is only the escape of dissolved air (or other gases), in the manner of the bubbling of fizzy drinks. It is especially liable to happen when there is a sudden release of pressure.23

  23. For a discussion of bubbling, see De Luc 1772, 2:380-381, §1033.

  end p.20

  Now which of these is "true" boiling? None of the options is palatable, and none can be ruled out completely, either. Bubbling would not seem to be boiling at all, but as we will see in "The Understanding of Boiling" section, a popular later theory of boiling regarded boiling as the release of water vapor (gas) dissolved in liquid water. Hissing and fast evaporation can probably be ruled out easily enough as "boiling" as we know it, since in those cases no bubbles of vapor come from within the body of water through to its surface; however, we will see in "A Dusty Epilogue" that there was a credible theoretical viewpoint in which evaporation at the surface was regarded as the essence of "boiling." Probably closest to De Luc's original conception of "true ebullition" is bumping (and explosion as a special case of it), in which there is little or no interference by dissolved air and the "first layer" of water is probably allowed to reach something like saturation by heat. But defining bumping as true boiling would have created a good deal of discomfort with the previously accepted notions of the boiling point, since the temperature of bumping is not only clearly higher than the temperature of common boiling but also unstable in itself. The only remaining option was to take common boiling as true boiling, which would have implied that the boiling point was the boiling temperature of impure water, mixed in with air. In the end, De Luc seems to have failed to reach any satisfactory conclusions in his investigation of boiling, and there is no evidence that his results were widely adopted or even well known at the time, although there was to be a powerful revival of his ideas many decades later as we will see shortly.

  In the course of the nineteenth century, further study revealed boiling to be an even more complex and unruly phenomenon than De Luc had glimpsed. A key contribution was made in the 1810s by the French physicist-chemist Joseph-Louis Gay-Lussac (1778-1850). His intervention was significant, since he was regarded as one of the most capable and reliable experimenters in all of Europe at the time, and his early fame had been made in thermal physics. Gay-Lussac (1812) reported (with dubious precision) that water boiled at 101.232°C in a glass vessel, while it boiled at 100.000°C in a metallic vessel. However, throwing in some finely powdered glass into the glass vessel brought the temperature of the boiling water down to 100.329°C, and throwing in iron filings brought it to 100.000°C exactly. Gay-Lussac's findings were reported in the authoritative physics textbook by his colleague Jean-Baptiste Biot (1774-1862), who stressed the extreme importance of ascertaining whether the fixed points of thermometry were "perfectly constant." Biot (1816, 1:41-43) admitted that Gay-Lussac's phenomena could not be explained by the thermal physics of his day, but thought that they contributed to a more precise definition of the boiling point by leading to the specification that the boiling needed to be done in a metallic vessel. If Gay-Lussac and Biot were right, the members of the Royal Society committee had got reasonably fixed results for the boiling point only because they happened to use metallic vessels. The reasons for that choice were not explained in their reports, but De Luc may have advised the rest of the committee that his troublesome superheating experiments had been carried out in glass vessels.

  Gay-Lussac's results, unlike De Luc's, were widely reported and accepted despite some isolated criticism. However, it took another thirty years for the next significant step to be taken, this time in Geneva again, by the professor of physics

  end p.21

  François Marcet (1803-1883)—son of Alexandre, the émigré physician in London, and Jane, the well-known author of popular science. Marcet (1842) produced superheating beyond 105°C in ordinary water, by using glass vessels that had contained strong sulphuric acid; clearly, somehow, the acid had modified the surface in such a way as to make boiling more difficult. Superheating became a clearly recognized object of study after Marcet's work, stimulating a string of virtuoso experimental performances vying for record temperatures. François Marie Louis Donny (1822-?), chemist at the University of Ghent, combined this insight on adhesion with a revival of De Luc's ideas about the role of air and produced a stunning 137°C using airless water in his own special instrument. Donny declared: "The faculty to produce ordinary ebullition cannot in reality be considered as an inherent property of liquids, because they show it only when they contain a gaseous substance in solution, which is to say only when they are not in a state of purity" (1846, 187-188).

  In 1861 the work of Louis Dufour (1832-1892), professor of physics at the Academy of Lausanne, added yet another major factor for consideration. Dufour (1861, esp. 255 ) argued that contact with a solid surface was the crucial factor in the production of ebullition and demonstrated the soundness of his idea by bringing drops of water floating in other liquids up to 178°C, without even purging the air out of the water. Even Dufour was outdone, when Georg Krebs (1833-1907) in 1869 achieved an estimated 200°C with an improvement of Donny's technique.24

  The superheating race must have been good fun to watch, but it also presented a great theoretical challenge.25 All investigators now agreed that the raising of temperature to the "normal" boiling point was not a sufficient condition to produce boiling. What they could not agree on was what the additional conditions needed for the production of boiling were. And if these additional conditions were not met, it was not clear how far superheating could go. Donny in 1846 had already expressed bemused uncertainty on this point: "[O]ne cannot predict what would happen if one could bring the liquid to a state of perfect purity." Krebs, in the work cited earlier, opined that water completely purged of air could not boil at all. In the more careful view of Marcel Émile Verdet (1824-1866), renowned professor of physics in Paris credited with introducing thermodynamics to France, there was probably a limit to the degree of superheating, namely that point at which there is enough heat to vaporize the whole mass of water instantly. Verdet, however, admitted that there was only one experiment in support of that view, namely the now-classic work of Charles Cagniard de la Tour (1777?-1859) on the critical point, a tem
perature above which a gas cannot be liquefied regardless of pressure.26 There was sufficient uncertainty on this question even toward the end

  24. Krebs's work is reported in Gernez 1875, 354.

  25. An intriguing parallel might be drawn between this superheating race and the modern-day race to reach higher and higher temperatures in superconductivity.

  26. See the review of the works on superheating given in Gernez 1875. Donny's statement is quoted on p. 347, and the report of Verdet's view can be found on p. 353.

  end p.22

  of the nineteenth century. In 1878 the 9th edition of the Encyclopaedia Britannica reported: "It has been stated that the boiling of pure water has not yet been observed."27

  Escape from Superheating

  The discussion in the last section leaves a puzzle: if there were such unmanageable and ill-understood variations in the temperatures required for the boiling of water, how could the boiling point have served as a fixed point of thermometry at all? It seems that superheating would have threatened the very notion of a definite "boiling point," but all the thermometers being used for the investigation of superheating were graduated with sharp boiling points that agreed increasingly well with each other. The philosopher can only conjecture that there must have been an identifiable class of boiling phenomena with sufficiently stable and uniform temperatures, which allowed the calibration of thermometers with which scientists could go on to study the more exotic instances. Fortunately, a closer look at the history bears out that philosophical conjecture. There were three main factors that allowed the boiling point to be used as a fixed point despite its vagaries.

  First of all, an immediate relief comes in realizing the difference between the temperature that water can withstand without boiling, and the temperature that water maintains while boiling. All observers of superheating from De Luc onward had noted that the temperature of superheated water went down as soon as steady boiling was induced (or each time a large bubble was released, in the case of bumping). Extreme temperatures were reached only before boiling set in, so the shocking results obtained by Donny, Dufour, and Krebs could be disregarded for the purpose of fixing the boiling point. De Luc got as far as 112°C without boiling, but the highest temperature he recorded while the water was boiling was 103°C. Still, the latter is 3°C higher than the "normal" boiling temperature, and there was also Gay-Lussac's observation that the temperature of boiling water was 101.232°C in a glass vessel. Marcet (1842, 397 and 404) investigated this question with more care than anyone else. In ordinary glass vessels, he observed the temperature of boiling water to range from 100.4 to 101.25°C. In glass treated with sulphuric acid, the temperature while boiling went easily up to 103 or 104°C and was very unsteady in each case due to bumping.

  That is where the second factor tending to stabilize the boiling point enters. In fact, this "second factor" is a whole set of miscellaneous factors, which might cause embarrassment to misguided purists. The spirit of dealing with them was to do whatever happened to prevent superheating. I have already mentioned that the Royal Society committee avoided superheating by using metallic vessels instead of glass. Gay-Lussac had shown how to prevent superheating in glass vessels by

  27. "Evaporation," Encyclopaedia Britannica, 9th ed., vol. 8 (1878), 727-733, on p. 728; emphasis original. This entry was by William Garnett.

  end p.23

  throwing in metal chippings or filings (or even powdered glass). Other investigators found other methods, such as the insertion of solid objects (especially porous things like charcoal and chalk), sudden localized heating, and mechanical shocks. But in most practical situations the prevention of superheating simply came down to not bothering too much. If one left naturally occurring water in its usual state full of dissolved air (rather than taking the trouble to purge air out of it), and if one left the container vessels just slightly dirty or rough (instead of cleaning and smoothing it off with something like concentrated sulphuric acid), and if one did not do anything else strange like isolating the water from solid surfaces, then common boiling did take place. Serious theoretical arguments about the factors that facilitate ebullition continued into the twentieth century as we will see in the next section, but all investigators agreed sufficiently on how to break superheating and prevent bumping in practice. Verdet observed that under "ordinary conditions," there would be dissolved air in the water and the water would be in contact with solid walls, and hence boiling would "normally" set in at the normal boiling point (see Gernez 1875, 351). It was a great blessing for early thermometry that the temperature of boiling was quite fixed under the sort of circumstances in which water tended to be boiled by humans living in ordinary European-civilization conditions near the surface of the earth without overly advanced purification technologies.

  However, happy-go-lucky sloppiness is not the most robust strategy of building scientific knowledge in the end, as the Royal Society committee realized quite well. The committee's lasting contribution, the last of our three factors contributing to the fixity of the boiling point, was to find one clear method of reducing the variations of the boiling point due to miscellaneous causes. The following was the committee's chief recommendation: "The most accurate way of adjusting the boiling point is, not to dip the thermometer into the water, but to expose it only to the steam, in a vessel closed up in the manner represented," shown in figure 1.3 (Cavendish et al. 1777, 845). Somehow, using the boiled-off steam rather than the boiling water itself seemed to eliminate many of the most intractable variations in the temperature: The heat of the steam therefore appears to be not sensibly different in different parts of the same pot; neither does there appear to be any sensible difference in its heat, whether the water boil fast or slow; whether there be a greater or less depth of water in the pot; or whether there be a greater or less distance between the surface of the water and the top of the pot; so that the height of a thermometer tried in steam, in vessels properly closed, seems to be scarce sensibly affected by the different manner of trying the experiment. (824)

  The recommendation to use steam came most strongly from Cavendish (1776, 380), who had already made the same proposal in his review of the instruments used at the Royal Society. The committee report only noted that using steam did in fact produce more stable results, but Cavendish went further and gave theoretical reasons for preferring steam, in an unpublished paper that followed but modified Figure 1.3. A scheme of the metallic pots used for fixing the boiling point by the Royal Society committee on thermometry. In the figure mM is the thermometer, and E is the "chimney" for the escape of steam. ABCD in the separate figure is a loose-fitting tin plate to cover the chimney in just the right way. These are fig. 4 and fig. 3 of Cavendish et al. 1777, from the plate opposite p. 856. The full description of the vessel and its proper employment can be found on 845-850. Courtesy of the Royal Society.

  end p.24

  De Luc's theoretical ideas.28 Cavendish stated as the first two of his four "principles of boiling": Water as soon as it is heated ever so little above that degree of heat which is acquired by the steam of water boiling in vessels closed as in the experiments tried at the Royal Society, is immediately turned into steam, provided that it is in contact either with steam or air; this degree I shall call the boiling heat, or boiling point. It is evidently different according to the pressure of the atmosphere, or more properly to the pressure acting on the water. But 2ndly, if the water is not in contact with steam or air, it will bear a much greater heat without being changed into steam, namely that which Mr. De Luc calls the heat of ebullition. (Cavendish [n.d.] 1921, 354)

  Cavendish believed that the temperature of boiling water was variable, probably always hotter than the temperature of the steam, but to different degrees depending on the circumstances. The boiling water itself was not fixed in its temperature, and

  28. The essay, entitled "Theory of Boiling," is undated, but it must have been composed or last modified no earlier than 1777, since it refers to the result of the work of th
e Royal Society committee. Cavendish had studied De Luc's work carefully, as documented by Jungnickel and McCormmach 1999, 548, footnote 6.

  end p.25

  he thought that "steam must afford a considerably more exact method of adjusting the boiling point than water" (359-360).

  De Luc disagreed. Why would the temperature of boiled-off steam be more stable and universal than the temperature of the boiling water itself? In a letter of 19 February 1777 to Cavendish, written in the midst of their collaboration on the Royal Society committee, De Luc commented on Cavendish's "Theory of Boiling" and laid out some of his doubts.29 The following passage is most significant: Setting aside for a moment all theory, it seems that the heat of the vapor of boiling water can be considered only with difficulty as more fixed than that of the water itself; for they are so mixed in the mass before the vapor emerges that they appear to have no alternative but to influence the temperature of each other. So to suppose that the vapor at the moment it emerges has in reality a fixed degree of temperature of its own, it is necessary that it be rigorously true, and demonstrated through some immediate experiments, that the vapor in reality can be vapor only at this fixed degree of heat. [But] I do not find that this proceeds from your reasoning. … (De Luc, in Jungnickel and McCormmach 1999, 547 and 550)