Chap, xxxvi.] TEMPERATURE. 465 



expansion occurs. It is this that causes bursting of 

 water-pipes in times of frost. The pipe is burst by 

 the enormous force capable of being exerted by the 

 freezing water, but the burst is not then observed 

 because the water is frozen. It is when the period of 

 thaw arrives and the ice becomes liquefied that the 

 burst is declared. 



All the permanent gases expand almost to the 

 same extent for every degree of elevation of tempera- 

 ture. This amount is called their coefficient of expan- 

 sion. It amounts to -^r^rd f the volume (whatever 

 be the volume) for every degree centigrade, or xf^th 

 of the volume for every degree Fahrenheit. 



Thus, given the same pressure, a volume of gas at 

 C. will be increased by 10 x -^-g f its volume at 

 10 C. This is expressed by formula in the following 

 way : Suppose Yo to be the volume of a gas at C., 

 and it is required to find its volume (V) at t C., 

 then V = Yo (1 + at), a is the coefficient of expan- 

 sion, which, as we have seen, is -^~, or, expressed in 

 decimals, '00366. So the equation becomes 



V = Vo (1 + -00366 t}. 



Suppose the volume to be 1 litre (1000 cc.) at 0, 

 what is the volume at 100 C. 1 



V = 1000 (1 + -00366 x 100). 



The temperature of a body is a " quantity which 

 indicates how hot or how cold the body is." The 

 fact of expansion indicates a method by means of 

 which differences of temperature may be measured, 

 Thus, if we take a flask of air with a coloured index, 

 as described on page 469, and if we find that with a 

 constant pressure the index now rises and now falls, 

 this will indicate that the air is now expanding, now 

 contracting, that, in fact, it is being subjected to differ- 

 ences of temperature ; and the extent of the variations 

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