192 SCIENCE OF COMMON THINGS. 



What is liquefaction 1 Why ice is melted by heat. 



senta a metallic bar, fixed at one end, B, but left free at the other, and in 

 contact with the end of a pointer, K, moving freely over a graduated scale. 

 If the bar be heated by the flame of alcohol, the metal expands, and 

 pressing upon the end of the pointer moves it, in a greater or less degree. 



13 GQ On what principle have pyrometers generally been constructed f 



On the relative expansion of bars of iron, or some 

 other metal. 



12Q7 Does a thermometer inform us how much heat any body con- 

 tains ? 



No ; it merely points out a difference in the tem- 

 perature of two or more substances. All we learn by 

 the thermometer is whether the temperature of one 

 body is greater or less than that of another ; and if 

 there is a difference it is expressed numerically 

 namely, by the degrees of the thermometer. 



It must be remembered that these degrees are parts of an arbitrary 

 scale, selected for convenience, without any reference whatever to the 

 actual quantity of heat present in bodies. 



1368 After the expansion of solids, when acted upon by heat, what 

 other effect is next observed ? 



They change their original state, become liquid, or 

 melt. Many of them become soft before melting, so 

 that they may be kneaded ; for instance, wax, glass, 

 and iron ; in this condition, glass can be bent and 

 moulded like wax, and iron can be forged or welded. 



13 B9 What is meant by liquefaction ? 



The conversion of a solid into a liquid by the agency 

 of heat, as solid ice is converted into water by the heat 

 of the sun. 



137O Why is ice melted by the heat of the sun f 



Because, when the heat of the sun enters the solid 

 ice, it forces its particles asunder, till their attraction 

 of cohesion is sufficiently overcome to convert the solid 

 ice into a liquid. 



13*71 Why are metals melted by the heat of fire f 



Because, when the heat of the fire enters the solid 

 metal, it forces its particles asunder, till their attraction 

 of cohesion is sufficiently overcome to convert the solid 

 metal into a liquid. 



