More on experiment 4, and related phenomena
By the late 19th century, "boiling chips" were commonly used by engineers and physical chemists to facilitate smoother boiling; the idea can be traced back to Gay-Lussac, who reported a significant lowering of temperature in glass vessels effected by throwing in metal filings or even powdered glass.
Initially I experimented with relatively old-style boiling chips, namely marble "anti-bumping granules". These clearly lower the temperature of vehement boiling in glass and ceramic vessels and bring it close to 100°C, also making the bubbles smaller and more frequent. However, in metallic vessels, the granules had much less effect: in one case (2 August 2004), the addition of the granules to a Teflon-coated pot was even seen to raise the temperature by about 0.3°C. The amount of lowering depends on the amount of granules used. The following table summarises the effect of using a generous amount of granules in various vessels, with data taken from the best set of back-to-back trials I made (from 3 August). Temperatures are typically not stable, and the number listed is the maximum temperature observed in each case. Vigorous boiling actually starts at lower temperatures than those listed here.
With the use of PTFE (Teflon) boiling stones, the lowering of the boiling point is even more pronounced, with vehement boiling possible from as low as 98.2°C (observed on 25 August 2004, in a glass beaker). Bubbles form with great ease on the stones; while the water is boiling, the stones are continually covered in a film of vapour. The following table shows data from trials on 20 August; the effect of boiling stones does depend on the amount used, and it is not clear that the maximum effect was reached in these trials, except in the case of the glass beaker.
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