EXPANSION OF GASES. 783 



into its original bulk. If l cc of air be heated from 

 to 100, the pressure remaining constant at 760 mm and 

 the air therefore being allowed to expand, it expands 

 by iff, and the volume at 100 becomes l^ cc . But 

 if l^y-g- 00 of air be compressed into the bulk of l cc the 

 pressure of the gas, according to Mariotte's law, in- 

 creases in the proportion of 1 to l^yf , and its actual 

 amount may be found by the proportion 



: 760 mm : x 



x = 760 x 1038 mm -4. 



The specific gravity of gases varies considerably 

 with the temperature, in consequence of their rate of 

 expansion, and the influence of these changes cannot be 

 disregarded as it often may be in the case of solids and 

 liquids ; for even within the ordinary range of natural 

 changes of temperature the specific gravity of gases 

 may vary as much as several tenths of the amount ; 

 and it is also obvious from what has been stated that 

 the pressure under which a gas has been experimented 

 upon when its specific gravity was determined must 

 be distinctly stated when the result of the experiment 

 is given. In Table I. (at the end of this volume) the 

 specific gravity of some of the most important gaseous 

 bodies is given, the temperature being 0, and the pres- 

 sure that of a column of mercury 760 mm high. 



The following calculation is an example of the ap- 

 plication of the preceding principles. A litre (1000 CC ) 

 of air, at and 760 mm pressure, weighs 1*293 gramme. 



When heated to 15 it expands 1000 x JA = 54 CC *945, 







and the volume becomes 1054 CC *945. A quantity of 





