Heat 115 



boil in the open air, it is still hot enough to boil when the 

 air pressure is partially removed. It is because of this 

 that milk is evaporated in a vacuum for canning; it 

 is not necessary to make it so hot that it will be greatly 

 changed by the heat, if the boiling is done in a vacuum. 

 On a high mountain the slight air pressure lets the water 

 boil at so low a temperature that it never becomes hot 

 enough to cook food. 



Application 30. Two college students were short of money 

 and had to economize greatly. They got an alcohol lamp to 

 use in cooking their own breakfasts. They planned to boil 

 their eggs. 



" Let's boil the water gently, using a low flame," one 

 said ; " we'll save alcohol." 



" It would be better to boil the eggs fast and get them 

 done quickly, so that we could put the stove out altogether," 

 the other replied. 



Which was right? 



Application 31. Two girls were making candy. They 

 put a little too much water into it. 



" Let us boil the candy hard so that it will candy more 

 quickly," said one. 



" Why, you wasteful girl," said the other. " It cannot 

 get any hotter than the boiling point anyhow, so you can't 

 cook it any faster. Why waste gas? " 



Which girl was right? 



Inference Exercise 

 Explain the following : 



161. Warm air rises. 



162. The lid of a teakettle rattles. 



163. Heating water makes a steam engine go. 



164. When an automobile with good springs and without shock 



absorbers goes over a rut, the passengers do not get a jolt, 

 but immediately afterward bounce up into the air. 



