EXPANSION AND COMPRESSION OF GASES. 



13 



Liquid and gaseous bodies are both termed Fluids, 

 but the latter class is not capable of forming drops like 

 the former. If placed in a vessel which is not closed 

 they escape into space. The mass of air which surrounds 

 the earth to a height of many miles is called the atmo- 

 sphere, and common air is hence often called atmospheric 

 air. Bodies like air have neither a definite form nor 

 a definite volume. The smoke which issues from a 

 chimney, or from a cigar, is seen to spread out and to 

 increase its volume continually, until, in consequence of 

 this expansion, it becomes too rarefied to be discerned 

 by the eye. This smoke is a body like air, rendered 

 visible by an admixture of fine soot and other particles 

 which are carried up by the ascending gas. The 

 alteration in volume may be easily observed also in 

 pure atmospheric air. A glass retort, such as is used 

 for physical and chemical purposes, is partly filled with 

 water and inverted (fig. 13). By sucking at the 



FIG. 13 (I real size}. 



FIG. 14 (\recd size}. 



aperture a the water rises at b and sinks at c : the air 

 above c expands. But by firmly closing the lips round 



