CHAPTER XV. 



RELATION BETWEEN THE VOLUME AND TEMPERATURE 

 OF A GIVEN MASS OF GAS. 



31. COEFFICIENT OF EXPANSION OF AIR. 



i. Coefficient of expansion of a gas. Obtain a piece of thermometer 

 tubing of about 1 mm. bore and 20 cm. long. Suck into it a length of about 

 1 cm. of mercury. Seal one end of the tube and arrange that the index 

 of mercury comes near the middle of the tube when the end has been closed 

 and the tube is cool. Fasten the tube to a thermometer, closed end down- 

 wards (Fig. 68). There is in the tube a certain volume of air, and its volume 

 at different temperatures can be found as with liquids. Place the combined 

 thermometer and tube in melting ice and notice the position of the air 

 column with reference to the thermometer scale. Repeat the operation 

 for every 10 up to 100 C., taking care that the air column is immersed 

 completely in each case, and giving the tube two or three taps before making 

 an observation, in order to make sure that the mercury is not sticking to 

 the tube. Record the observations thus : 



As the tube is cylindrical and uniform in bore, the volume of the air in 

 it is proportional to the lengths of the air column. The average increase 

 of volume for 1 C., expressed as a fraction of the volume at C., is the 

 coefficient of expansion. Find from the results the coefficient of expansion 

 of air. 



When a gas is heated in circumstances where, as in these experiments, 

 free expansion is possible, it is said to expand under a constant pressure. 

 Both at the beginning of the experiment and after the gas has been heated 

 the pressure to which it is subjected is simply that of the atmosphere. 



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