38 COMMON PHENOMENA DUE TO HEAT 



plication of the fact that hard boiling water is not any hotter 

 than gently boiling water enables us to save considerable gas 

 and money. Suppose, for example, that three burners of a 

 gas range are being used to boil three separate pots. As soon 

 as the three vessels are all boiling, turn out two gas jets com- 

 pletely, and remove the third vessel from the flaming burner. 

 Then place over the lighted burner a metal plate large enough 

 to hold ail three pots. There will be enough heat from the 

 one burner to cause gentle boiling in all three vessels, and 

 the food will cook as quickly as though all three burners were 

 using gas. 



Heat necessary to change boiling water to steam. If 

 the flame which supplies heat is removed from boiling water, 

 the boiling ceases ; if the flame is replaced, boiling begins 

 again and some water passes off as steam. Unless heat is 

 constantly supplied, water at the boiling point cannot be 

 transformed into steam. 



By experiment it has been found that 536 calories of heat 

 are needed to change I gram of water at the boiling point into 

 steam. 



Statements similar to the above hold not only for water but also 

 for other liquids. If milk is placed upon a stove, the tempera- 

 ture rises steadily until the boiling point is reached ; further heat- 

 ing does not produce a change in temperature, but a change of the 

 water of the milk into steam. As soon as milk, or any other 

 liquid food, comes to a boil, the gas flame should be lowered 

 until only an occasional bubble forms, because so long as any 

 bubbles form the temperature is that of the boiling point, and 

 further heat merely results in waste. Every liquid has its own 

 specific boiling point ; fo r example, alcohol boils at 78.4 C. 

 and water at 100 C. 



Condensation. If a cold lid is held in the steam of boiling 

 water, drops of water gather OR the lid ; the steam is cooled 



