CHAPTER III 

 COMMON PHENOMENA DUE TO HEAT 



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 bottom of the kettle and 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 tempera- 

 ture 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 reading 

 becomes stationary at about 212 F. 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 gas is turned low, the water boils less hard. If a ther- 

 mometer is now placed in the water, the temperature registers 

 212 F. just as it did when the water was boiling hard. This 

 fact enables the thrifty person to economize gas in cooking. 

 As soon as soup, vegetables, or other substances come to a boil, 

 the gas can be partly turned off, and only sufficient used to 

 keep the food gently boiling. It is the heat which cooks the 

 food, and since gently boiling water is just as hot as briskly 

 boiling water, why should extra gas be used for that which 

 yields no return ? And yet, how many housewives think 

 to lower the gas as soon as the pot begins to boil ? The ap- 



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