Your explanation is, however, also flawed. It is not mainly the heating of the air before the jar is placed over the candle which causes trouble but the heating after this, which expands the air inside the jar, forcing bubbles out under the rim. On cooling back to room temperature, there is therefore much less air in the jar. A second, equally serious, flaw is the assumption that the candle will continue to burn until all the oxygen in the air has been consumed. It will, in fact, cease to burn when the oxygen content has fallen from its normal 21 per cent to about 15 per cent.
A valid experiment of this type which overcomes both the above problems and gives an approximately correct result involves igniting yellow phosphorus under a bell-jar. It was demonstrated to me as an 11-year-old 45 years ago but few schools take the trouble to do it now.
All three main sciences have their jokers-in-the-pack among school experiments. Having disposed of the jar-and-candle, perhaps the Fist children would like to take on the balloons-in-a-bell-jar, which purportedly shows what causes the lungs to inflate on inhalation, and the physics teachers' "spouting jar" which is erroneously supposed to prove that water pressure increases with the height of a column of water.
DEREK HASLAM MSC
Colne, LancashireReuse content