FRICTION AND COMPRESSION PRODUCE HEAT 47 



EXPERIMENTS FOR THE TEACHER 



Use a thermometer with large index, and show the high 

 temperature produced by pouring sulphuric acid into water 

 (use care) ; try hydrochloric acid and zinc, sulphuric acid and 

 iron; mix equal parts of potassium chlorate and sugar to- 

 gether, place on iron pan, and add a few drops of sulphuric 

 acid, at arm's length. Add a small piece of metallic sodium 

 to water. The hydrogen, set free, ignites spontaneously. 



32. FRICTION AND COMPRESSION PRODUCE HEAT 



When one body rubs against another, there is a certain re- 

 sistance which depends upon the force of contact and the 

 surface quality of the bodies. We call the cause of this 

 resistance friction. Since the molecules which are on the sur- 

 faces of the two bodies are caused to move more rapidly, heat 

 is produced. Examples : production of fire by the rubbing 

 together of two sticks, and also by the use of flint and steel. 



If we hammer a piece of lead, we force the molecules to slip 

 by one another and give them an added velocity, which 

 becomes apparent as heat. Thus there can be external and 

 internal friction, but the result is always a production of heat. 



Since molecular motion is heat, we should expect that if we 

 could increase the number of molecules within a given space, 

 the temperature would rise. The only way in which this 

 object can be accomplished is by compression. If we com- 

 press a gas, it becomes hotter. Conversely, if the gas is allowed 

 to expand, it becomes cooler. Refrigerating plants and ice 

 machines are founded on this principle. Ammonia gas is 

 compressed and then is allowed to expand in the desired 

 place, which lowers the temperature of that place consider- 



