HEAT. 43 



2. HEAT. 



Motion of molecules. If we place over a gas-flame a vessel con- 

 taining a lump of ice of the temperature of C., or 32 F., the ice 

 melts and becomes converted into water ; but if we measure with a 

 thermometer the temperature of the water at the moment when the 

 last particle of ice is melted, we still find it at the freezing-point, 

 C. or 32 F. From the position of the vessel over the flame, as 

 well as from .the fact that the ice has been liquefied, we know that 

 the vessel and its contents have absorbed heat. Yet vessel and water 

 show the same temperature as before. If the heat of the flame is 

 allowed to continue its action on the ice-cold water, the thermometer 

 will soon indicate a rapid absorption of heat until the temperature 

 reaches 100 C. or 212 F. Then the water begins to boil and 

 escapes in the form of steam, but the temperature remains stationary 

 until the last particle of water has disappeared. 



There must be, consequently, some relation between the state of 

 aggregation of a substance and that agent which we call heat. It was 

 the heat which liquefied the ice, it was the heat which converted the 

 liquid water into steam or gaseous water. Yet the water, having 

 absorbed considerable heat during the process of melting, shows a 

 temperature of C. (32 F.), and the steam also having absorbed 

 large quantities of heat, shows 100 C. (212 F.), the temperature of 

 boiling water. A certain amount of heat has consequently been lost 

 or at least hidden. What has become of it? 



To answer this we must first examine a little further into the 

 nature of heat. It is a well-known fact that when two solid bodies 

 are rubbed together heat is produced. Ice may be melted and water 

 boiled by friction ; wood may be made to ignite by rubbing it suita- 

 bly. It is found by accurate experiments that there is an intimate 

 relation between the amount of heat generated and the amount of 

 work done to generate it. The amount of heat is always equivalent 

 to the amount of work expended. This fact is one of the manifesta- 

 tions of the law known as the law of the correlation of energy. 



Many other illustrations of production of heat by the expenditure 

 of work could be given, and all would point to the conclusion that 

 heat is associated in some way with the condition of the small parti- 

 cles of which substances are made up i. e., the molecules. 



It is believed that the molecules of all bodies are in motion. 

 Those of gases are perfectly free to move in any direction, while in 

 liquids and solids they are restricted in their motion. In solids the 



