TEilEESTHIAL HEAT. 



A fluid so light and mobile as the atmosphere, can never remain 

 long in a state of repose, and the column of air suspended over 

 the surface of any collection of water however extensive, is suhject 

 to frequent change. In general, therefore, before any such 

 portion of the atmosphere becomes saturated by evaporation, it is 

 removed and replaced by another portion. It happens, conse- 

 quently, that the atmosphere rarely becomes saturated by the 

 immediate effect of evaporation. 



62. The state of saturation is, however, often attained either by 

 loss of temperature, or by the intermixture of strata of air of 

 different temperatures and differently charged with vapours. 

 Thus, if air which is below the point of saturation suffer a loss of 

 heat, its temperature may fall to that point which is the highest 

 compatible with the density of the vapour actually suspended in it. 

 The air will then become saturated, not by receiving any increased 

 quantity of vapour, but by losing that caloric by which the vapour 

 it contained was previously superheated. 



If two strata of air at different temperatures, and both charged 

 with vapour to a point below saturation, be intermingled, they 

 will take an intermediate temperature, that which had the higher 

 temperature imparting a portion of its heat to that which had a 

 lower temperature. The vapour with which they were previously 

 charged will likewise be intermixed and reduced to the common 

 temperature. Now, in this case it may happen that the common 

 temperature to which the entire mass is reduced, after inter- 

 mixture, shall be either equal to or less than the greatest 

 temperature compatible with the density of the vapour in the mass 

 of air thus mixed. If it be equal to that temperature, the mass 

 of air after intermixture will be saturated, though the strata 

 before intermixture were both below saturation ; and if less, con- 

 densation must take place until the density of the vapour 

 suspended in the mixture be reduced to the greatest density 

 compatible with the temperature. 



It might be supposed that air and vapour being mixed together 

 without combining chemically, would arrange themselves in 

 strata, the lighter floating above the heavier as oil floats above 

 water. This statical law, however, which prevails in liquids, is 

 in the case of elastic fluids subject to important qualifications. 

 The latter class of fluids have a tendency to intermingle and 

 diffuse themselves through and among each other in opposition to 

 their specific gravities. Thus if a stratum of hydrogen, the lightest 

 of the gases, rest upon a stratum of carbonic acid, which is the 

 heaviest, they will by slow degrees intermingle, a part of the 

 hydrogen descending among the carbonic acid, and a part of the 

 carbonic acid ascending among the hydrogen, and this will continue 

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