Scientific Lectures. 63 



bubbles of vapor rising and escaping from the surface of the fluid. 

 An egg might be boiled all day in water at 120 deg, without being 

 cooked, because it requires a greater heat to cook it. As these little 

 bubbles rise in the tea-kettle, they strike a colder stratum of water 

 and are condensed, the water falling to fill the vacuum, producing the 

 sound we call the singing of the tea-kettle. The next stage of our 

 process of boiling will be the process of distillation, which consists in 

 the transfer of particles of water out of the liquid state into vapor, 

 then its translation and final recondensation in another place. [Prof, 

 S. here illustrated the process of distillation.] The amount of heat 

 passing into the water in the tea-kettle would be measured by the 

 thermometer until it reached 212 deg. At that point the thermometer 

 would cease to rise, although the heat was still passing 'as rapidly as 

 before into the water, the surplus being employed in converting the 

 water into steam, which escapes from the vessel. Having heated water 

 in a glass vessel to the boiling point, we remove the fire and cork it up. 

 It continues to boil ; and upon pouring cold water upon the surface, 

 it boils still more violently. Wliy ? Because the condensation of 

 the steam removes the pressure, and the water boils more readily, 

 even at a lower temperature, [He proceeded to try Count Eumford's 

 experiment, of building a hot fire, with a temperature of not less 

 than 2,000 deg. above a vessel of water.] The surface of the water 

 boiled, as shown by its condensation upon a cold glass plate laid 

 above it ; but the water in the vessel was not heated. It is necessary, 

 therefore, to heat the tea-kettle at the bottom, and not at the top. 

 If we desire to boil substances which will be injured by the tempera- 

 ture of 212 deg,, we may readily boil them at any lower temj)erature, 

 above 100 deg., by removing the pressure of the atmosphere. Taking 

 equal quantities, by weight, of ice at 32 deg., and boiling water at 

 212 deg., the ice was melted by the water, and the temperature of 

 the mixture was 52 deg. There had disappeared 140 deg, of heat, 

 and this was the latent heat, without which the water would remain 

 ice. Every one has noticed that the melting of ice in spring causes 

 a great chill in the atmosphere ; for whenever and wherever ice is 

 melted, it absorbs inevitably 140 deg. of heat. On the other hand, 

 the vaporization of water takes up a great deal of heat, which is 

 rendered latent ; for steam itself, at the pressure of the atmosphere, 

 has only a temperature of 212 deg. If we measure the heat thus 

 becoming latent, we shall find that it amounts to about 970 deg. By 

 adding constantly a given quantity of heat, we shall find that it takes 



