EXPANSION. 57 



hesion to overcome, as in the two other states of 

 matter. While iron increases in general bulk ^y^th. 

 and water about ^Vd, on being heated from the freez- 

 ing to the boiling point of the latter, air expands more 

 than Jd by the same increase of temperature. 



126. LAW OF EXPANSION FOR GASES. 



State the law 



of expansion Gases expand ^oth of the bulk which they 

 for gases. possess at 32, for every degree above that 

 point, and contract in the same proportion for every de- 

 gree below it. Thus, 490 cubic inches at 32 would 

 so expand as to occupy an inch more space at 3-3, 

 still another inch at 34, and at the same rate for 

 higher temperatures. And the same quantity would 

 contract by cold, or withdrawal of heat, so as to oo 

 cupy an inch less space at 31, and two inches less at 

 30, and so on for lower temperatures. The law is 

 the same for steam and other vapors. 

 What is a 127. THE THERMOMETER. The ther- 

 thermometer ? niometer is an instrument in which ex- 

 pansion is made use of to show changes of tem- 

 perature. A straight wire, which would grow regu- 

 larly and perceptibly longer in proportion to the 

 increase of temperature, would form the most conve- 

 nient thermometer. But solids do not expand enough, 

 or with sufficient regularity, for this purpose. The 

 liquid metal mercury, is therefore employed instead, 

 being inclosed in a glass tube and bulb. 



128. MANUFACTURE OF THERMOME- 



How are ther- 



mometcrsman- TERs. In making thermometers, the 



mercury being first introduced into the 



bulb, is boiled, so as to expel all air and moisture, 



3* 



