VOLUME AND TEMPERATURE OF GAS 



85 



Measurement of expansion. While a definite rise of temperature 

 causes most bodies to expand, the amount of the expansion varies 

 within wide limits. In the case of certain special alloys it is almost 

 negligible, while gases expand more than double their volume on being 

 heated from d to 300 C. 



Having now a means at command of measuring temperatures, a 

 more exact comparison of expansions becomes possible. The coeffi- 

 cient of expansion is the term used for the rate of expansion when the 

 temperature is increased. In the case of solids we are usually concerned 

 with the linear coefficient of expansion, while for liquids and gases the 

 coefficient of cubical expansion is of importance. 



The fraction of its volume at C., which a gas expands when its tem- 

 perature is raised through 1 C. is called its coefficient of cubical expansion. 



Expansion of gases. The expansion of gases is very much greater 

 than that of either solids or liquids. Thus, 273 c.c. of dry air at C. 

 become 274 c.c. at 1 C. and 373 c.c. at 100 C. 

 The coefficient of expansion is consequently 2^5 > 

 and this is practically the coefficient for all gases. 

 This law is not, however, strictly obeyed by all 

 gases, though air and some other gases conform to 

 it with great accuracy. The air thermometer is 

 often used as a standard, since the expansion of 

 air with increase of temperature is both regular 

 and large. 



The coefficient of cubical expansion of a gas can 

 be determined by enclosing a definite quantity of 

 air in a long narrow tube closed at one end, the 

 gas being separated from the air by a tiny thread 

 of mercury (Fig. 68). The length of the column 

 of air represents the original volume of the gas. 

 The expansion the air undergoes will push the 

 index outwards, and these movements can be 

 observed. If temperature observations are simul- 

 taneously taken, the expansion per unit volume 

 per degree of temperature can be obtained. 



Normal temperature and pressure. It should 

 now be evident that the volume of a given mass 

 of gas depends on the temperature at which it is measured, and also 

 (section 27) on the barometric pressure at the time of measurement. 

 This being the case, certain standard conditions for taking measure- 

 ments of gases have been decided upon, viz. C. and a pressure 

 equal to that of a column of mercury 760 mm. in height. In practice 



