EXPANSION AND CONTRACTION 



occupies more space than formerly, and since the quantity of air 

 has not changed, each cubic inch of space must hold a less 

 weight of warm air than it held of cold air ; that is, one cubic 

 inch of warm air weighs less than one cubic inch 

 of cold air, or, warm air is less dense than cold 

 air. All gases which are not confined expand 

 when heated and contract as they cool. Heat, 

 as a general rule, causes substances to expand 

 or become less dense. 



Amount of expansion and contraction. While 

 most substances expand when heated and con- 

 tract when cooled, they are not all affected 

 equally by the same changes in temperature. 

 Alcohol expands more than water, and water 

 more than mercury. Steel wire which measures 

 J mile on a snowy day gains 25 inches in length 

 on a warm summer day, and an aluminum wire 

 under the same conditions gains 50 inches in 

 length. 



Advantages and disadvantages of expansion and contraction. 

 We owe the snug fit of metal tires and bands to the expansion 

 and contraction resulting from heating and cooling. The tire 

 of a wagon wheel is made slightly smaller than the wheel which it 

 is to protect ; it is then put into a very hot fire and heated until 

 it has expanded sufficiently to slip on the wheel. As the tire 

 cools it contracts and fits the wheel closely. 



In building a railroad, spaces are usually left between consecu- 

 tive rails in order to allow for expansion during the summer. 



The unsightly cracks and humps in cement floors are some- 

 times due to the expansion resulting from heat (Fig. 4). Crack- 

 ing from this cause can frequently be avoided by cutting the soft 

 cement into squares, the spaces between them giving opportunity 

 for expansion just as do the spaces between the rails of railroads. 



FIG. 3. 



air in A is> heated, 

 it expands and 

 forces the drop of 

 ink up the tube. 



