310 



TEMPERATURE. 



energy of these molecular motions. Molecules thus vi- 

 brating must push each other further apart, and thus cause 

 the body which they constitute to expand. This expansion, 

 or increase of volume, is the first effect of heat upon 

 bodies. 



(a.) Imagine, if possible, twenty-five quiet boys standing closely 

 crowded together. Upon the floor draw a chalk line enclosing the 

 group. If these boys be suddenly set shaking, as by the ague, they 

 will force some of their number over the chalk line. From the 

 motions of the individuals has resulted an expansion of the living 

 mass. 



484. Expansion Illustrated. The expansion of 

 solids may be shown by a ball, which, at ordinary tempera- 

 tures, will easily pass through a 

 ring ; on heating the ball it will 

 no longer pass through the ring. 

 If the ball be cooled by plung- 

 ing it into cold water, it will 

 again pass through the ring. 

 This illustrates the increase of 

 volume or cubical expansion. 

 Sometimes the expansion in 

 length only is measured. This 

 is called linear expansion. Ex- 

 pansion is also illustrated in the 



FIG. 241. compensation pendulum ( 149). 



485. Unequal Expansion. Different substances 

 expand at different rates for the same change of temper- 

 ature. This may be shown by heating a bar made by 

 riveting together, side by side, two thin bars of equal size, 

 one of iron and one of brass, so that the compound bar 

 shall be straight at the ordinary temperature. As brass 



