Experiment 5: Slow heating
Here we have water being heated on a hotplate, in a volumetric flask with a long, thin neck. This is an approximation of De Luc’s attempts to use slow heating in order to bring the whole body of water to the temperature as that of the first layer in contact with the heat source. The hotplate is very hot but still much gentler than a naked flame. (The behaviour of the water in this setup is very different from boiling driven by an open flame. As the temperature approaches 100°C, the water starts to boil in a normal way. As boiling continues, however, the temperature continues to rise, while the bubbles get bigger but less frequent; they also come more irregularly, often in bursts. The temperature goes over 100°C, easily reaching 101-102°C while the boiling is reasonably steady. This is what 19th-century observers termed "bumping".) Later in the process we can observe the "puffing" behavior. With continued heating, the bubbles can become even less frequent, while temperature creeps up further. Often there are long quiet periods punctuated by isolated large bubbles. Sometimes the puffs are explosive, throwing some water out of the flask. The trial shown in this clip produced a temperature of 104°C during puffing. All of this is entirely consistent with what De Luc had reported in 1772. The increasing “bumpiness” of boiling is most likely the result of having more and more dissolved air taken out of the water, as I discuss later in the explanation of "de-gassing" process involved in Experiment 6.
Watch a video clip showing this experiment (1 min. 50 sec.)
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