Is the steam point more fixed than the boiling point?

The 1777 report of the Royal Society Committee only stated 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fact produce more stable results, but Cavendish went further and gave theoretical reasons for preferring steam, in an unpublished paper that followed but modified De Luc's theoretical ideas. As the first two of his four "principles of boiling" Cavendish stated:

"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 excess of the heat of water above the boiling point is influenced by a great variety of circumstances." ([n.d.] 1921, 359) The boiling water itself was not fixed in its temperature, and he thought that "steam must afford a considerably more exact method of adjusting the boiling point than water." (ibid., 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. 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)

As De Luc's doubts underscore, Cavendish's preference for steam rested on an extraordinary belief: that steam emerging from water boiling under a fixed pressure must always have the same temperature, whatever the temperature of the water itself may be. This claim required a defence.

First of all, why would steam (or, water vapor) emerging out of water be only at 100°C, if the water itself has a higher temperature? Cavendish's answer was the following:

"These bubbles [of steam] during their ascent through the water can hardly be hotter than the boiling point; for so much of the water which is in contact with them must instantly be turned into steam that by means of the production of cold thereby, the coat of water . . . in contact with the bubbles, is no hotter than the boiling point; so that the bubbles during their ascent are continually in contact with water heated only to the boiling point." (Cavendish [n.d.] 1921, 359)

The fact that the formation of steam requires a great deal of heat was widely accepted since the work of the celebrated Scottish physician and chemist Joseph Black (1728-1799) on latent heat, and De Luc and Cavendish had each developed similar ideas, though they recognized Black's priority. However, for a convincing demonstration that the steam in the body of superheated boiling water would always be brought down to the "boiling point," one needed a quantitative estimate of the rate of cooling by evaporation in comparison with the amount of heat continually received by the steam from the surrounding water. Such an estimate was well beyond Cavendish's reach.

De Luc also asked what would prevent the steam from cooling down below the boiling temperature, after emerging from the boiling water. Here Cavendish offered no theoretical account, except a dogmatically stated principle, which he considered empirically vindicated: "steam not mixed with air as soon as it is cooled ever so little below the [boiling temperature] is immediately turned back into water." ([n.d.] 1921, 354) De Luc's experience was to the contrary: "when in a little vessel, the mouth and its neck are open at the same time, the vapor, without condensing, becomes perceptibly cooler." He also doubted that steam could realistically be obtained completely free of air, which was necessary in order for Cavendish's principle to apply at all (See De Luc, in Jungnickel and McCormmach 1999, 549-550).

Judging from the final report of the Royal Society Committee, it is clear that no firm consensus was reached on this matter. While the Committee's chief recommendation for obtaining the upper fixed point was to adopt the steam-based procedure advocated by Cavendish, the report also approved two alternative methods, both using water rather than steam and one of them clearly identified as the procedure from De Luc's previous work. (See Cavendish et al. 1777, 832, 850-853 for reference to De Luc's previous practice.) De Luc's acquiescence on the chief recommendation was due to the apparent fixedness of the steam temperature, not due to any perceived superiority in Cavendish's theoretical reasoning. In his February 1777 letter to Cavendish mentioned above, there were already a couple of indications of De Luc's surprise that the temperature of steam did seem more fixed than he would have expected. De Luc exhorted, agreeably:

"Let us then, Sir, proceed with immediate tests without dwelling on causes; this is our shortest and most certain path; and after having tried everything, we will retain what appears to us the best solution. I hope that we will finally find them, by all the pains that you wish to take." (De Luc, in Jungnickel and McCormmach 1999, 550)

Thus the use of steam enabled the Royal Society Committee to obviate divisive and crippling disputes about theories of boiling. It gave a clear operational procedure that served well enough to define an empirically fixed point, though there was no agreed understanding of why steam coming off boiling water under a given pressure should have a fixed temperature.

A more decisive experimental confirmation of the steadiness of the steam temperature had to wait for 65 years until Marcet's work, in which it was shown that the temperature of steam was nearly at the standard boiling point regardless of the temperature of the boiling water from which it emerged. Even when the water temperature was over 105°C, the steam temperature was only a few tenths of a degree over 100°C (Marcet 1842, 404-405). That is still not negligible, but it was a reassuring result given that it was obtained with serious superheating in the boiling water. The situation was much ameliorated by the employment of all the various means of converting superheated boiling into common boiling. If these techniques could bring the water temperature down fairly close to 100°C, then the steam temperature would be reliably fixed at 100°C, or very near it. Marcet's work closed a chapter in the history of superheating in which it posed a real threat to the fixity of the boiling point, although it did not address the question of whether steam could cool down below the boiling point.

The "steam point" proved its robustness. After the challenge of superheating was overcome in the middle of the 19th century, the fixity (or rather, the fixability) of the steam point did not come into any serious doubt. Marcet already stated in 1842 that the steam point was "universally" accepted. The remaining difficulty now was in making sense of the empirically demonstrated fixability of the steam point. This task of understanding was a challenge that interested all but the most positivistic physicists. (Read more about the 19th-century theories of boiling.)

However, if we jump ahead a bit in history, we actually find a serious doubt being raised about the actual stability of the steam point. This came in the form of the late 19th-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 his own way and by his own methods." These qualities, as we will see, are amply demonstrated in his work on steam and water. (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.)

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 wasn't sufficient dust around. Aitken showed this in a very simple experiment (ibid., 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. . . ." (ibid., 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. 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? 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.

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