A. Boiling is the effect of a more violent escape of air from the heated water; when, therefore, the air is not permitted to escape, water will never boil.
Q. Why is heat applied to the bottom, and not to the top of a kettle?
A. Because the heated water always ascends to the surface, heating the water through which it passes: if, therefore, heat were applied to the top of a vessel, the water below the surface would never be heated.
Q. As the lower part of a grate is made red-hot by the fire above, why would not the water boil, if fire were applied to the top?
A. The iron of a grate is an excellent conductor; and, therefore, if one part be heated, the heat is conducted to every other part: but water is a very bad conductor, and will not diffuse heat in a similar way.
Q. How do you know that water is a bad conductor of heat?
A. When a blacksmith immerses his red-hot iron in a tank of water, the water which surrounds the red-hot iron is made boiling hot, but the water below the surface remains quite cold.
Q. If you wish to cool liquids, where should the cold be applied?
A. To the top of the liquid; because the cold portion will always descend, and allow the warmer parts to come in contact with the cooling substance.
Q. Does boiling water get hotter by being kept on the fire?