CHAPTER III 



OTHER FACTS ABOUT HEAT 



20. Boiling. Heat absorbed in Boiling. If a kettle of 

 water is placed above a flame, the temperature of the water 

 gradually increases, and soon small bubbles form at the bot- 

 tom of the kettle and begin to rise through the water. At 

 first the bubbles do not get far in their ascent, but disappear 

 before they reach the surface ; later, as the water gets hotter 

 and hotter, the bubbles become larger and more numerous, 

 rise higher and higher, and finally reach the surface and 

 pass from the water into the air ; steam comes from the vessel, 

 and the water is said to boil. The temperature at which a 

 liquid boils is^ called the boiling point. 



While the water is heating, the temperature steadily rises, 

 but as soon as the water begins to boil the thermometer read- 

 ing becomes stationary and does not change, no matter how 

 hard the water boils and in spite of the fact that heat from 

 the flame is constantly passing into the water. 



If the flame is removed from the boiling water for but a 

 second, the boiling ceases ; if the flame is replaced, the boil- 

 ing begins again immediately. Unless heat is constantly 

 supplied, water at the boiling point cannot be transformed 

 into steam. 



The number of calories which imist be supplied to i gram of 

 water at tJie boiling point in order to change it into steam at 

 the same temperature is called tJie Jieat of vaporization ; it is 

 the heat necessary to change I gram of water at the boiling 

 point into steam of the same temperature. 



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