Temperature. Thermometers 155 



gas does not alter during the expansion, since alteration 

 of pressure would cause change of volume on its own 

 account and thus might counteract or augment the 

 effect properly due to the heating alone. Dalton in 

 England and Charles in France independently dis- 

 covered this regularity in the expansions of gases and 

 the statement given above is variously known as 

 Charles's or Dalton's Law. 



98. Examples of the use of the law in calculations 

 involving expansion or contraction of gases under the 

 influence of heat may be best worked as below : 



(a) How much air will escape from a room 5 

 metres long, 3 metres high, and 4- 5 metres wide, when 

 the average temperature of the room changes from 

 10 to 21 ? 



Interior volume of room = 5 x 3 x4'5 = 67*5 cu. m. 

 Each cubic metre at 10 will have at a volume of 

 |Jf cu. m. Since the increase in volume of each cu. m. 

 for a rise of 1 is ^{3 of its volume at 0, each cu. m. 

 at 10 will gain -^ x fjf cu. m. for each rise of 1. 

 The total increase in volume will therefore be 



67'5 x 11 x jjjy = 2-62 cu. m., 



i.e. this volume of air will escape from the room. 



(6) At what temperature will 100 cubic feet of 

 coal-gas measured at 15, become 101 cu. ft. ? 



By a similar line of argument to that used in the 

 former example, it may be shown that each cu. ft. at 

 15 gains in volume by ^ cu. ft. for each rise of 1. 

 Hence the rise in temperature is (101 100) -f- 

 = 2*88 and the required temperature is 15 + 2*88 

 = 17-88. 



99. Let us next consider the decrease in volume 

 of 273 c.c. of a gas at when its temperature is lowered 



