Inventing Temperature Read online

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  water without the joint action of alien molecules, can only produce its change of state well beyond what is considered the temperature of normal ebullition.

  Dufour's notion was that the production of vapor would only take place when a sort of equilibrium that maintains the liquid state was broken. Boiling was made possible at the point of pressure balance, but some further factor was required for the breaking of equilibrium, unstable as it may be. Heat alone could serve as the further facilitating factor, but only at a much higher degree than the normal boiling point. Dufour also made the rather subtle point that the vapor pressure itself could not be a cause of vapor production, since the vapor pressure was only a property of "future vapor," which did not yet exist before boiling actually set in. Dufour's critique was cogent, but he did not get very far in advancing an alternative. He was very frank in admitting that there was insufficient understanding of the molecular forces involved.36 Therefore the principal effect of his work was to demolish the adhesion hypothesis without putting in a firm positive alternative.

  There were two main attempts to fill this theoretical vacuum. One was a revival of Cavendish's and De Luc's ideas about the importance of open surfaces in enabling a liquid to boil. According to Cavendish's "first principle of boiling," the conversion of water at the boiling point into steam was only assured if the water was in contact with air or vapor. And De Luc had noted that air bubbles in the interior of water would serve as sites of vapor production. For De Luc this phenomenon was an annoying deviation from true boiling, but it came to be regarded as the definitive state of boiling in the new theoretical framework, which I am about to explain in further detail.

  One crucial step in this development was taken by Verdet, whose work was discussed briefly in the last section. Following the basic pressure-balance theory, he defined the "normal" point of boiling as the temperature at which the vapor pressure was equal to the external pressure, agreeing with Dufour that at that temperature boiling was made "possible, but not necessary." Accepting Dufour's view that contact with a solid surface was a key factor promoting ebullition, Verdet also made an interesting attempt to understand the action of solid surfaces along the Cavendish-De Luc line. He theorized, somewhat tentatively, that boiling was not provoked by all solid surfaces, but only by "unwettable" surfaces that also possessed microscopic roughness. On those surfaces, capillary repulsion around the points of irregularity would create small pockets of empty space, which could serve as sites of evaporation. There would be no air or steam in those spaces initially, but it seemed sensible that a vacuum should be able to serve the same role as gaseous spaces in enabling evaporation. If such an explanation were tenable, then not only Dufour's observations but all the observations that seemed to support the adhesion hypothesis could be accounted for.37

  Verdet's idea was taken up more forcefully by Désiré-Jean-Baptiste Gernez (1834-1910), physical chemist in Paris, who was one of Louis Pasteur's "loyal

  36. For this admission, see the last pages of Dufour 1861, esp. 264.

  37. In the exposition of Verdet's view, I follow Gernez 1875, 351-353.

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  Figure 1.5. Gernez's instrument for introducing an open air surface into the interior of water. Courtesy of the British Library.

  collaborators" and contributed to various undertakings ranging from crystallography to parasitic etiology.38 In articles published in 1866 and 1875, Gernez reported that common boiling could always be induced in superheated water by the insertion of a trapped pocket of air into the liquid by means of the apparatus shown in figure 1.5. A tiny amount of air was sufficient for this purpose, since boiling tended to be self-perpetuating once it began. Gernez (1875, 338) thought that at least half a century had been wasted due to the neglect of De Luc's work: "[T]he explanation of the phenomenon of boiling that De Luc proposed was so clear and conformable to reality, that it is astonishing that it was not universally adopted."39 In Gernez's view a full understanding of boiling could be achieved by a consistent and thorough application of De Luc's idea, a process initiated by Donny, Dufour, and Verdet among others. Donny (1846, 189) had given a new theoretical definition of boiling as evaporation from interior surfaces: "[B]oiling is nothing but a kind of extremely rapid evaporation that takes place at interior surfaces of a liquid that surrounds bubbles of a gas."

  Gernez (1875, 376) took up Donny's definition, adding two refinements. First, he asserted that such boiling started at a definite temperature, which could be called "the point of normal ebullition." He added that the gaseous surfaces within the liquid could be introduced by hand or produced spontaneously by the disengagement of dissolved gases. (Here one should also allow a possible role of Verdet's

  38. The information about Gernez is taken from M. Prévost et al., eds., Dictionnaire de Biographie Française, vol. 15 (1982), and also from comments in G. Geison's entry on Pasteur in the Dictionary of Scientific Biography, 10:360, 373-374.

  39. In Gernez's view, the general rejection of De Luc's ideas had probably been prompted by De Luc's "unfortunate zeal" in opposing Lavoisier's chemistry.

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  empty spaces created by capillary forces and of internal gases produced by chemical reactions or electrolysis.40 ) Gernez's mopping up bolstered the pressure-balance theory of boiling quite sufficiently; the presence of internal gases was the crucial enabling condition for boiling, and together with the balance of pressure, it constituted a sufficient condition as well. The theoretical foundation of boiling now seemed quite secure.

  There were, however, more twists to come in the theoretical debate on boiling. While Verdet and Gernez were busily demonstrating the role of gases, a contrary view was being developed by Charles Tomlinson (1808-1897) in London. Tomlinson believed that the crucial enabling factor in boiling was not gases, but small solid particles. Tomlinson's argument was based on some interesting experiments that he had carried out with superheated liquids. Building on previous observations that inserting solid objects into a superheated liquid could induce boiling, Tomlinson (1868-69, 243) showed that metallic objects lost their vapor-liberating power if they were chemically cleaned to remove all specks of dust. In order to argue conclusively against the role of air, he lowered a small cage made out of fine iron-wire gauze into a superheated liquid and showed that no boiling was induced as long as the metal was clean. The cage was full of air trapped inside, so Tomlinson inferred that there would have been visible production of vapor if air had really been the crucial factor. He declared: "It really does seem to me that too much importance has been attached to the presence of air and gases in water and other liquids as a necessary condition of their boiling" (246). Defying Dufour's warning against theorizing about boiling on the basis of the properties of "future vapor," Tomlinson started his discussion with the following "definition": "A liquid at or near the boiling-point is a supersaturated solution of its own vapour, constituted exactly like soda-water, Seltzer-water, champagne, and solutions of some soluble gases" (242). This conception allowed Tomlinson to make use of insights from his previous studies of supersaturated solutions.

  Tomlinson's theory and experiments attracted a good deal of attention, and a controversy ensued. It is not clear to me whether and how this argument was resolved. As late as 1904, the second edition of Thomas Preston's well-informed textbook on heat reported: "The influence of dissolved air in facilitating ebullition is beyond question; but whether the action is directly due to the air itself or to particles of dust suspended in it, or to other impurities, does not seem to have been sufficiently determined" (Preston 1904, 362). Much of the direct empirical evidence cited by both sides was in fact ambiguous: ordinary air typically contained small solid particles; on the other hand, introducing solid particles into the interior of a liquid was likely to bring some air into it as well (as De Luc had noticed when he tried to insert thermometers into his air-free water). Some experiments were less ambiguous, but still not decisive. For example, Gernez acknowl
edged in his attack on Tomlinson that the latter's experiment with the wire-mesh cage would clearly be negative evidence regarding the role of air; however, he claimed that Tomlinson's result could not be trusted because it had not been replicated by anyone else. Like

  40. The latter effect was demonstrated by Dufour 1861, 246-249.

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  Dufour earlier, Gernez (1875, 354-357, 393) also scored a theoretical point by denigrating as unintelligible Tomlinson's concept of a liquid near boiling as a supersaturated solution of its own vapor, though he was happy to regard a superheated liquid as a supersaturated solution of air.

  The Tomlinson-Gernez debate on the theory of boiling is fascinating to follow, but its details were in one clear sense not so important for the question we have been addressing, namely the understanding of the fixity of the steam point. Saturated vapor does obey the pressure-temperature relation, whatever the real cause of its production may be. Likewise, the exact method by which the vapor is produced is irrelevant as well. The pressure-temperature relation is all the same, whether the vapor is produced by steady common boiling, or by bumpy and unstable superheated boiling, or by an explosion, or by evaporation from the external surface alone. After a century of refinement, then, it became clear that boiling itself was irrelevant to the definition or determination of the "boiling point."

  A Dusty Epilogue

  If the determination of the boiling point hinged on the behavior of steam both theoretically and experimentally, we must consider some key points in the physics of steam, before closing this narrative. Most crucial was the definite relationship between the pressure and the temperature of saturated steam. After witnessing the tortuous debates in the apparently simple business of the boiling of water, would it be too rash to bet that there would have been headaches about the pressure-temperature relation of saturated steam, too?

  Recall De Luc's worry that saturated steam might cool down below the temperature indicated by the pressure-temperature law without condensing back into water, despite Cavendish's assertion that it could not. In the particular setup adopted by the Royal Society committee for fixing the boiling point, this probably did not happen. However, in more general terms De Luc's worry was vindicated, a whole century later through the late nineteenth-century investigations into the "supersaturation" of steam. This story deserves some brief attention here, not only because supersaturation should have threatened the fixity of the steam point but also because some insights gained in those investigations threw still new light on the understanding of boiling and evaporation.

  The most interesting pioneer in the study of supersaturation, for our purposes, was the Scottish meteorologist John Aitken (1839-1919). His work has received some attention from historians of science, especially because it provided such an important stimulus for C. T. R. Wilson's invention of the cloud chamber. Aitken had trained as an engineer, but abandoned the career soon due to ill health and afterwards concentrated on scientific investigations, mostly with various instruments that he constructed himself. According to his biographer Cargill Knott, Aitken had "a mind keenly alive to all problems of a meteorological character," including the origin of dew, glacier motion, temperature measurement, the nature of odorous emanations, and the possible influence of comets on the earth's atmosphere. He was a "quiet, modest investigator" who refused to accept "any theory which seemed to him insufficiently supported by physical reasoning," and studied every problem "in

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  his own way and by his own methods." These qualities, as we will see, are amply demonstrated in his work on steam and water.41

  Aitken was explicit about the practical motivation for his study of supersaturated steam: to understand the various forms of the "cloudy condensations" in the atmosphere, particularly the fogs that were blighting the industrial towns of Victorian Britain (1880-81, 352). His main discovery was that steam could routinely be cooled below the temperature indicated by the standard pressure-temperature relation without condensing into water, if there was not sufficient dust around. Aitken showed this in a very simple experiment (338), in which he introduced invisible steam from a boiler into a large glass receptacle. If the receptacle was filled with "dusty air—that is, ordinary air," a large portion of the steam coming into it condensed into small water droplets due to the considerable cooling it suffered, resulting in a "dense white cloud." But if the receptacle was filled with air that had been passed through a filter of cotton wool, there was "no fogging whatever." He reckoned that the dust particles served as loci of condensation, one dust particle as the nucleus for each fog particle. The idea that dust was necessary for the formation of cloudy condensations obviously had broad implications for meteorology. To begin with: If there was no dust in the air there would be no fogs, no clouds, no mists, and probably no rain. … [W]e cannot tell whether the vapour in a perfectly pure atmosphere would ever condense to form rain; but if it did, the rain would fall from a nearly cloudless sky. … When the air got into the condition in which rain falls—that is, burdened with supersaturated vapour—it would convert everything on the surface of the earth into a condenser, on which it would deposit itself. Every blade of grass and every branch of tree would drip with moisture deposited by the passing air; our dresses would become wet and dripping, and umbrellas useless. … (342)

  The implication of Aitken's discovery for the fixity of the steam point is clear to me, though it does not seem to have been emphasized at the time. If steam can easily be cooled down below the "steam point" (that is, the temperature at which the vapor pressure of saturated steam equals the external pressure), the steam point is no more fixed than the boiling point of liquid water. Moreover, what allows those points to be reasonably fixed in practice is precisely the same kind of circumstance: the "ordinary" conditions of our materials being full of impurities—whether they be air in water or dust in air. Cavendish was right in arguing that steam would not go supersaturated, but he was right only because he was always dealing with dusty air.42 Now we can see that it was only some peculiar accidents of human life that gave the steam point its apparent fixity: air on earth is almost always dusty enough, and no one had thought to filter the air in the boiling-point apparatus. (This role of

  41. The fullest available account of Aitken's life and work is Knott 1923; all information in this paragraph is taken from that source, and the quoted passages are from xii-xiii. For an instructive discussion of Aitken's work in relation to the development of the cloud chamber, see Galison 1997, 92-97.

  42. What saved Cavendish could actually be the fact that in his setup there was always a water-steam surface present, but that raises another question. If a body of steam is in contact with water at one end, does that prevent supersaturation throughout the body of the steam, however large it is?

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  serendipity in the production of stable regularities will be discussed further in "The Defense of Fixity" in the analysis part of this chapter.)

  Before I close the discussion of Aitken's work, it will be instructive to make a brief examination of his broader ideas. Not only do they give us a better understanding of his work on the supersaturation of steam but also they bear in interesting ways on the debates about boiling that I have discussed in previous sections. His work on the supersaturation of steam came from a general theoretical viewpoint about the "conditions under which water changes from one of its forms to another." There are four such changes of state that take place commonly: melting (solid to liquid), freezing (liquid to solid), evaporation (liquid to gas), and condensation (gas to liquid). Aitken's general viewpoint about changes of state led him to expect that steam must be capable of supersaturation, before he made any observations of the phenomenon.43 In his own words: I knew that water could be cooled below the freezing-point without freezing. I was almost certain ice could be heated above the freezing-point without melting. I had shown that water could be heated above the boiling-point. … Arrived at this point, the presumption was very strong that wate
r vapour could be cooled below the boiling-point … without condensing. It was on looking for some experimental illustration of the cooling of vapour in air below the temperature corresponding to the pressure that I thought that the dust in the air formed 'free surfaces' on which the vapour condensed and prevented it getting supersaturated. (Aitken 1880-81, 341-342)

  Changes of state are caused by changes of temperature, but "something more than mere temperature is required to bring about these changes. Before the change can take place, a 'free surface' must be present." Aitken declared: When there is no 'free surface' in the water, we have at present no knowledge whatever as to the temperature at which these changes will take place. … Indeed, we are not certain that it is possible for these changes to take place at all, save in the presence of a "free surface." (339)