The experiments leave a puzzle regarding the very possibility of thermometry: if there were such unmanageable and ill-understood variations in the temperatures of boiling water, how could the boiling point have served as a fixed point of thermometry at all? It seems that the variations would have threatened the very notion of a definite "boiling point", but the very thermometers used for the investigation of the variations were graduated with sharp boiling points. 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 then go on to study the more exotic instances. Happily, 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). 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 (this latter figure is also quite consistent with my own experimental results). 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 over 101°C in glass vessels. Marcet (1842, 397 and 404) investigated this question with more care than anyone else at the time. In ordinary glass vessels, he observed the temperature of boiling water to range from 100.4° to 101.25°C. In glass treated with hot sulphuric acid, the temperature while boiling went easily up to 103° or 104°C, and was very unsteady in each case due to bumping. So, a further stabilizing factor was needed.

The second factor tending to stabilize the boiling point was in fact a whole set of miscellaneous factors, which might cause embarrassment to misguided purists. The spirit here 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 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 many 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 hot 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 20th century, but all investigators agreed sufficiently on how to break superheating and prevent bumping in practice. Marcel 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" (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." (ibid., 824)

The recommendation to use steam came most strongly from Cavendish, who had already made the same proposal in his review of the instruments used at the Royal Society (Cavendish 1776, 380). But is the steam point really more fixed than the boiling-point? If so, why? (Read more about the fixedness of the steam point.)

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