More on experiment 6, and related phenomena

De Luc reported in 1772 that purging dissolved air out of water made it able to withstand very high temperatures without boiling (112°C in one trial), and theorized that the dissolved air facilitated boiling by forming small bubbles that function as sites of vapour-production (read more about early theories and modern theories of boiling).

On the hotplate the temperature of water in a long-necked flask will not reliably go above 104-105°C, because the occasional large bubbles will bring down the temperature each time. Higher superheating is only achieved with a cooler heat source. By burying the flask up to the neck in graphite, it is possible to boil the water in the flask with graphite temperature at just over 200°C, though usually brought it to around 250°C. With such gentle heating, it is easy to bring the water up to 108-109°C, before it boils off explosively and comes down to a somewhat lower temperature (but nowhere near 100°C). Afterwards the water may settle down to steadier (slightly superheated) boiling, or it may continue puffing and explosions. The latter is the likely outcome if the initial de-gassing is sufficient; the temperature tends to be unsteady, but can often be relatively stable if an equilibrium is reached between the heat influx from the graphite and the heat outflow by means of fast evaporation from the surface.

Once the water is de-gassed in a long-necked flask, it is quite difficult to make it boil normally again. The following (27 August 2004) is typical behaviour with good de-gassing, temperatures recorded with a Beckmann thermometer: initial explosion at 105.50°C, about 4 minutes after passing 100°C; post-explosion temperature down to 103.4°C; afterwards temperature climbing back up quite steadily, despite a couple of small sharp puffs; 4 minutes after the initial explosion, another explosion, at about 106.3°C; for 2.5 minutes afterwards, puffing at around 105.5°C; thermometer removed for the next 4.5 minutes, resulting in no significant difference in the behaviour of the water; on re-insertion of the thermometer, temperature found to be steadily over 106°C, with occasional puffs accompanied by a hissing noise.

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