TtfE PATH OF EVOLUTION 



of the molecules and their momentum produces a pres- 

 sure tending to force away the solid walls, which ten- 

 dency is counterbalanced by the pressure of the air on 

 the outside, if at normal atmospheric pressure, or by the 

 rigidity and tension of the retaining walls, if the in- 

 ward pressure is greater or less than the normal. The 

 effect of these movements is that the bulk or volume 

 of a gas of any composition is in the inverse ratio of 

 the pressure at the same temperature. If the vessel 

 is air-tight the bulk will diminish one-half by 

 doubling the pressure, or will increase proportionately 

 in volume if the confining pressure is reduced. If a 

 gas is not restrained by confinement in a closed vessel, 

 its molecules continue moving away in right lines in- 

 definitely. If we apply the above statements to the 

 gases of the atmosphere, we find that the atmosphere 

 (which exerts a pressure of about fifteen pounds to 

 the square inch) if viewed as of a uniform volume 

 and density must have an average height or depth of 

 about five miles ; but, as it expands in proportion as 

 the distance from the earth increases, being released 

 in part from the pressure of its own mass, it probably 

 reaches actually a height of 200 miles or more. No 

 means are known of correctly ascertaining its limit. 

 Its expansion may be ultimately checked by the ex- 

 treme cold of the upper aerial regions, which must 

 approximate or attain the absolute zero; it might 

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