100 APPLIED SCIENCE 



muriatic acid is added to zinc the chemical reaction which 

 takes place produces heat; a current of electricity passing 

 through a piece of platinum raises the temperature of the 

 platinum. 



Two very common effects of heat noticed in every-day 

 life are the changes in length, surface, or volume of mate- 

 rials, and the changes of state from solid to liquid and from 

 liquid to gaseous. Since heat is due to the motion of the 

 particles that compose a body, it will expand as the rate of 

 motion is increased. This principle is utilized when the 

 blacksmith first heats a tire before putting it on a wheel so 

 that when the tire contracts as it cools it fits closely. For 

 the same reason, rivets are made red-hot before they are 

 put into boilers, bridges, or steel structures. When cool they 

 contract and draw the parts tightly together. 



Heat travels in three distinct ways: by conduction, by 

 convection, and by radiation. 



When a poker is placed in a fire, the heat passes along 

 the poker from the hot to the cold part; this action illus- 

 trates conduction. Heat passes through some materials 

 more readily than through others; materials of the first 

 class are called good conductors and those of the second 

 class, poor conductors. Iron, for instance, is a good conductor 

 and wood a poor conductor of heat. 



The heat from a stove passes through the air without 

 any apparent motion; movement of heat in this manner is 

 called convection. 



Heat comes to us from the sun; this method of trans- 

 mission of heat is called radia-tion. 



101. The Manufacture of Thermometers. For the meas- 

 urement of modern temperatures there are two standard 



