De Luc's work on boiling
Jean-André de Luc's work on the boiling of water was reported in great detail in his Recherches sur les modifications de l'atmosphère (1772), vol. 2, pp. 227-438. De Luc started his work on boiling by investigating possible variations in the boiling temperature according to the "degree of boiling." Initially De Luc stated, in the first volume of his Recherches sur les modifications de l'atmosphère:
"When water begins to boil, it does not yet have the highest degree of heat it can attain. For that, the entire mass of the water needs to be in movement; that is to say, that the boiling should start at the bottom of the vessel, and spread all over the surface of the water, with the greatest impetuosity possible. From the commencement of ebullition to its most intense phase, the water experiences an increase in heat of more than a degree." (De Luc 1772, 1:351-352, §439)
In further experiments, De Luc showed that there was an interval of several degrees (95-100°C, or 203-212°F) corresponding to the spectrum of ebullition ranging from "hissing" to full boil, which is quite consistent with the range of 204-212°F indicated in Adams's thermometer discussed above.
His own less-than-perfect understanding of boiling evidently kept troubling De Luc. Just as his book was going to the press in 1772, De Luc added a 15-chapter supplement to his discussion of thermometers, entitled "inquiries on the variations of the heat of boiling water." The logical starting point of this research was to give a precise definition of boiling, before disputing whether its temperature was fixed. What, then, is boiling? De Luc (1772, 2:369, §1008) conceived "true ebullition" ("la vraie ébullition") as the phenomenon in which the "first layer" of water in contact with the heat source becomes saturated with the maximum possible amount of heat ("fire" in his terminology), thereby turning into vapor and rising up through the water in the form of bubbles. He wanted to determine the temperature acquired by this first layer.
That was a tall order experimentally, since the first layer was bound to be so thin that no thermometer could be immersed in it. Initial experiments revealed that there must indeed be a substantial difference between the temperature of the first layer and the rest of the water under normal conditions. For example, when De Luc heated water in a metallic vessel put into an oil bath, the thermometer in the middle of the water reached 100°C only when the oil temperature was 150°C or above. One could only surmise that the first layer of water must have been brought to a temperature somewhere between 100°C and 150°C. De Luc's best estimate, from an experiment in which small drops of water introduced into hot oil exploded into vapor when the oil was hot enough, was that the first layer of water had to be at about 112°C, for true ebullition to occur. (For further details on these experiments, see De Luc 1772, 2:356-362, §§980-993.)
Was it really the case that water could be heated to 112°C before boiling? Perhaps incredulous about his own results, De Luc devised a different experiment (1772, 2:362-364, §§994-995). Thinking that the small drops of water suspended in oil may have been too much of an unusual circumstance, in the new experiment he sought to bring all of a sizeable body of water up to the temperature of the first layer. To curtail heat loss at the open surface of the water, he put the water in a glass flask with a long narrow neck (only about 1cm wide) and heated it slowly in an oil bath. The water boiled in an unusual way, by producing very large occasional bubbles of vapor, sometimes explosive enough to throw off some of the liquid water out of the flask. While this strange boiling was going on, the temperature of the water fluctuated between 100°C and over 103°C. After some time, the water filled only part of the flask and settled into a more steady boil, at the temperature of 101.9°C. De Luc had observed what later came to be called "superheating," namely the heating of a liquid beyond its normal boiling point. (The term "superheating" was first used by John Aitken in the 1870s, as far as I can ascertain; see Aitken 1878, 282. The French term surchauffer was in use quite a bit earlier.)
It now seemed certain to De Luc that the temperature necessary for true ebullition was higher than the normally recognized boiling point of water. But how much higher? There was one major problem in answering that question. The presence of dissolved air in water induced an ebullition-like phenomenon before the temperature of true ebullition was reached. De Luc knew that ordinarily water contained a good deal of dissolved air, some of which was forced out by heating and formed small bubbles (often seen sticking to the inner surface of vessels), before the boiling point was reached. He was also well aware that evaporation from the surface of water happened at a good rate at temperatures well below boiling. Putting the two points together, De Luc concluded that significant evaporation must happen at the inner surfaces of the small air bubbles at temperatures much lower than that of true ebullition. Then the air bubbles would swell up with vapor, rise, and escape, releasing a mixture of air and water vapor. Does that count as boiling? It surely has the appearance of boiling, but it is not true ebullition as De Luc defined it. He identified this action of dissolved air as "the greatest obstacle" that he had to overcome in his research: "that is, the production of internal vapors, which is occasioned by this emergence of air, before there is true ebullition." (For the discussion of the role of air in boiling, see De Luc 1772, 2:364-368, §§996-1005; the quoted passage is from p. 364.)
De Luc was determined to study true ebullition, and that meant obtaining water that was completely purged of dissolved air. He tried everything. Luckily, sustained boiling actually tended to get much of the air out of the water. And then he filled a glass tube with hot boiled water and sealed the tube; upon cooling, the contraction of the water created a vacuum within the sealed tube, and further air escaped into that vacuum. This process could be repeated as often as desired. De Luc also found that shaking the tube (in the manner of rinsing a bottle, as he put it) facilitated the release of air; this is a familiar fact known to anyone who has made the mistake of shaking a can of carbonated beverage (For a description of the purging process, see De Luc 1772, 2:372-380, §§1016-1031.) After these operations, De Luc obtained water that entered a steady boil only in an oil bath as hot as 140°C. But as before, he could not be sure that the water had really taken the temperature of the oil bath, though this time the water was in a thin tube. Sticking a thermometer into the water in order to verify its temperature had the maddening side-effect of introducing some fresh air into the carefully purified water. There was no alternative except to go through the purging process with the thermometer already enclosed in the water, which made the already-delicate purging operation incredibly frustrating and painful. He reported:
"This operation lasted four weeks, during which I hardly ever put down my flask, except to sleep, to do business in town, and to do things that required both hands. I ate, I read, I wrote, I saw my friends, I took my walks, all the while shaking my water. . . ." (De Luc 1772, 2:387, §§1046-1049)
Four mad weeks of shaking had its rewards. The precious airless water he obtained could stand the heat of 97.5°C even in a vacuum, and under normal atmospheric pressure it reached 112.2°C before boiling off explosively (ibid., 2:396-397, §§1071-1072). The superheating of pure water was now confirmed beyond any reasonable doubt, and the temperature reached in this experiment was very much in agreement with De Luc's initial estimate of the temperature reached by the "first layer" of water in ebullition.
Superheating was an experimental triumph for De Luc. However, it placed him into a theoretical dilemma, if not outright confusion. Ordinary water was full of air and not capable of attaining true ebullition, but his pure airless water was not capable of normal boiling at all, only explosive puffing with an unsteady temperature. To complicate matters further, the latter type of boiling also happened in a narrow-necked flask even when the water had not been purged of air. De Luc had started his inquiry on boiling by wanting to know the temperature of true boiling; by the time he was done, he no longer knew what true boiling was. At least he deserves credit for realizing that boiling was not a simple, homogeneous phenomenon. The following is the phenomenology of what can happen to water near its boiling point, which I have gathered from various parts of De Luc's 1772 treatise. It is not a very neat classification, despite my best efforts to impose some order.
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.
4. 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.
5. 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.
Now which of the above is "true" boiling? None of the options are palatable, and none can be ruled out completely, either. Bubbling would not seem to be boiling at all, but a popular later theory of boiling, by Charles Tomlinson, 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 Section A6 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 (see the history of superheating).