784 EXPANSION OF GASES. 



water of the same bulk weighs 1054 CC '945 grammes, and 

 the specific gravity of air referred to water is conse- 



1 -903 



quently * , njr , = 0'001226. Let now the tempera- 

 1054*945 



ture remain constant and the pressure become 744 mm ; 



the bulk of the gas would then by Mario tte's law 



increase in the proportion of 744 to 760 ; that is 



744 : 760 :: 1054-945 :x 



x = 1077 cc -62. 



The volume of 1-293 gramme of air at 15 and 740 mm 

 pressure is thus 1077 CC> 62 ; and as the weight of an 

 equal volume of water is 107 7' 62 grammes, it follows 

 that the specific gravity of air under those conditions 



1*293 



of temperature and pressure is = 0'0012. The 



10 1 I '\)Z 



specific gravity of any gas at any temperature and 

 pressure may be similarly calculated, if its specific 

 gravity at and 760 mm pressure is known. 



Since gases expand more than liquids and their 

 specific gravity diminishes much more rapidly when 

 they are heated, the upward and downward move- 

 ments of the particles of a heated gas proceed at a much 

 quicker rate than in liquids. In the contrivance (fig. 

 200) represented on page 302, the ascent of heated 

 air is taken advantage of for giving motion to a 

 spiral of paper. In a room where a fire is kindled a 

 current of warm air begins to ascend immediately ; and 

 if the hot air be cooled by contact with the walls and 

 windows, a complete circulation of the air will be 

 maintained in the room, the hot air constantly ascend- 



