SCIENCE OF COMMON THINGS. 155 



How to cool liquids. Boiling point of liquids. 



999 As the lower part of a grate is made red hot by the fire above, why 

 would not the water boil if fire were applied to the top of a kettle ? 



The iron of a grate is an excellent conductor ; if, 

 therefore, one part be heated, the heat is conducted to 

 every other part ; but water is a very bad conductor, 

 and will not diffuse heat in a similar way. 



1000 If you wish to cool liquids, where should the cold be applied f 



To the top of th liquid / because the cold portions 

 will always descend, and allow the warmer part to come 

 in contact with the cooling substance. 



1001 Does boiling water get hotter by being kept on the fire ? 



No ; not if the steam be suffered to escape. 



1OOS Why does not boiling water get hotter if the steam be suffered to 

 escape f 



Because the water is converted into steam as fast as 

 it boils ; and the stearn carries away the additional 

 heat, 



1003 What is ebullition f 



"When a liquid substance is heated sufficiently to 

 form steam, the production of vapor takes place princi- 

 pally at that part where the heat enters ; and when the 

 heating takes place not from above, but from the bot- 

 tom and sides, the steam as it is produced rises in bub- 

 bles through the liquid, and produces the phenomenon 

 of boiling or ebullition. 



1004 What do we mean by the boiling point of a liquid ? 



The temperature at which vapor rises with sufficient 

 freedom to cause the phenomenon of ebullition is called 

 the boiling point. 



1005 Do att liquids boil at the same temperature f 



No ; the boiling point occurs in different liquids at 

 very different temperatures. 



1006 Why does milk 'boil over more readily than water f 



Because the bubbles of milk, produced by the pro- 

 cess of boiling, are more tenacious than the bubbles of 

 water ; and these bubbles, accumulating and climbing 

 one above another, seon overtop the rim of the saucepan 

 and run over. 



1007 Why does water simmer before it boils? 



