First I attempted de-gassing operations similar to the ones described by De Luc, using a vacuum and some shaking, which failed to make a noticeable difference in superheating. I was not willing to get into De Luc's four-week shaking regime, so I looked for another method (Aren't there worries about not using the same procedure used by the historical scientists?).

It is probable that there is already a good deal of de-gassing that happens simply by virtue of heating to the boiling temperature. The solubility-ratio of air in water under atmospheric pressure is 0.0258 at 40°F, and 0.0004 at 212°F, so only about 1/60 of the air dissolved in water near freezing would remain by the time it is heated to boiling temperature.

According to De Luc, the process of boiling itself also tends to take air out of the water (perhaps by air being swept out along with the bubbles of vapour, which is likely especially if air serves to facilitate bubble-nucleation). If this does happen, and de-gassing causes superheating, then we can understand why the temperature of boiling water creeps up while boiling is going on, as seen in Experiments 1 and 5.

So, the first step of the de-gassing procedure I employed was to boil water for some time (up to 30 minutes), in a loosely covered pot. The point of the loose covering is to keep pressure at normal level while filling the space above the water with steam rather than air, so as to minimize the re-introduction of air at the open water surface while the boiling takes place.

After this preliminary de-gassing, the water is poured carefully into a long-necked flask so that the water-line comes up into the neck and the surface area of water in contact with air is minimized. Heating this water on a hotplate results in very puffy boiling, with very large occasional bubbles and temperature climbing up during quiet phases. A volumetric flask also has the advantage of having a neck that is not only narrow but quite long, which means that while boiling is going on the neck would fill with vapour and air would have a difficult time getting through to the water. By the time the temperature reaches around 104°C and there are long quiet periods, the water is sufficiently de-gassed for the trial in the graphite bath.

I expect that the solubility of air goes down further as the water temperature is increased beyond 100°C. (I have not been able to find data on the solubility of air in superheated water.) So there seems to be a runaway effect here: degassing makes boiling difficult, resulting in superheating (since it is the formation of vapour that most effectively removes heat from boiling water); superheating reduces air-solubility, resulting in further degassing.

Return to the description of Exepriment 6

Return to the Experiments section of the main page

Return to the start of the main page