Our Surroundings 



We have prepared hydrogen gas by pouring hydrochloric 

 acid on pieces of zinc in a bottle. As the acid acted on the zinc 

 perhaps we noticed that the bottle became warm. This heat 

 came as the result of chemical action. An automobile tire, when 

 blown up hard, becomes a little warmer during the process. The 

 adding of more air to the inside of the tire increases the pres- 

 sure of the air, and this pressure produces heat. We turn on 

 the electric light and notice that the bulb becomes warm in a 

 very short time. We say that electricity produces heat. In saw- 

 ing a piece of lumber we notice that the saw becomes hot from 

 the friction. All of these different means of producing heat 

 illustrate the transformation of energy. 



According to the molecular theory of matter, all substances 

 are made up of very small particles called molecules. These mole- 

 cules are all in rapid vibration, even in heavy solids, like iron. 

 The rate of motion of these molecules varies according to the 

 heat of the body. As a body becomes warm it expands. 

 This is caused by the molecules moving more rapidly and 

 pushing each other farther apart, thus increasing the size of 

 the body. As the body cools, the motion of these same mole- 

 cules grows less, they draw closer together and the body shrinks 

 in size. 



When the builder places large rivets in a bridge to hold the 

 iron girders together, he uses them red hot so that when they 

 cool they will be shorter and so draw the parts more closely 

 together. When a wagon maker puts a steel tire on a wheel, 

 he makes it just large enough to go on the wheel when the tire 

 is hot, and when it cools it shrinks and grips the wheel very 

 tightly. 



Were you ever traveling by rail when your train was forced 

 to stop because of a "hot box"? This means that the axle was 

 rubbing harder than usual on the part with which it comes in 

 contact, called the journal box. Such friction produces heat. 

 The journal box is filled with a cotton material, called waste, to 

 hold a supply of oil, as oil reduces friction by making the parts 

 move easily. If the oil supply becomes low the waste sometimes 

 takes fire when the journal box gets hot. 



