152 HEAT. 



The bubbling of water when boiling, is owing to the 

 formation of steam, which rises continually from the 

 bottom to the top of the vessel. At the common pressure 

 of the air, near the level of the sea, water boils at 2 12 . 

 The higher we carry water up the side of a mountain, 

 the less heat is required to make it boil ; because a part 

 of the pressure of the atmosphere is removed. A differ- 

 ence of 1 in the boiling point of water corresponds to 

 a difference in elevation of nearly 520 feet. The heavier 

 the air is, the greater the pressure, and of course the 

 more heat is required to make water boil. By increasing 

 the pressure sufficiently, water may be heated to 400 

 without ebullition. The effect of pressure on the boiling of 

 water, may be shown by this experiment. Take a com- 

 mon olive oil flask, fit a good cork to it, put about half a gill 

 of water into it, and place it over a lamp until it boils. 

 Permit the boiling to continue a short time, and then 

 place the cork in the neck tight, and remove it from the 

 lamp ; the water will continue to boil a short time after. 

 On plunging the flask into cold water, the boiling will 

 again commence with violence. Remove the flask into 

 hot water and the boiling will cease ; and if it is again 

 plunged into cold water, it will again boil. 



The boiling of the water in this experiment, before the 

 cork is placed in it, expels the air in the bottle, and 

 steam takes its place ; and the flask when removed from 

 the lamp, becomes a little cooled, which condenses a 

 portion of the steam, which removes some of the pressure, 

 and thus causes the water still to boil. When the flask 

 is plunged into cold water, more of the steam is con- 

 densed, and more of the pressure removed, which causes 

 it to boil violently ; and when the bottle is placed in 

 warm water, a portion of steam is again formed, thereby 

 increasing the pressure, and thus prevents the boiling 

 which pressure is again removed by plunging the Mask 

 into cold water, and thus causes the water again to boil. 

 Steam imparts its great heat so easily to bodies which 

 are colder than itself, as to fit it peculiarly for many useful 

 practical purposes in the arts and in domestic life. The 

 heat produced by means of steam, for many purposes is 

 preferable to the heat of fire. Dyers' and brewers' vats 

 are much better heated by steam than in any other way. 



