278 THE MECHANICS OF THE EARTH's ATMOSPHERE. 



mixture iu this ratio before the subsequent dissolutiou. From this 

 value of ^3 this mixing ratio itself can be determined. 



We find by a very simple consideration that for this special value of 

 ^3 the following equation holds good : 





But since 



and 

 therefore 



and consequently 



F2Fi= 



tjr-ti 

 K 



FiF^=yi—yi 

 V\—y\ j^_U—t\ 



tz—u t2-h »h 



Whenever /^</.is: that is to say, when the cooler " mechanically sui^er- 

 saturated air," or at least the saturated component enters into the mix- 

 ture with greater weight, then will /</i and of course /</2, or in other 

 words, the finally resulting temperature will be lower than that of 

 either component. 



These considerations lead to the following apparently very paradox- 

 ical result: ^^ If warmer air is mixed with mechanically saturated or me- 

 chanically super-saturated air then a part of the susjwnded water can be 

 evaporated and thereby a lower temperature produced J^ 



" If the given mechanically saturated air is hygroscopicaUy unsaturated, 

 that is to say, if the vapor is unsaturated, then this loicering of tempera- 

 ture icill occur even by the mixing of saturated warmer air {of course in 

 the proper ratio) ; if the air is saturated as to its vapor and the corre- 

 sponding mechanical mixture is present as pure super-saturation^ then the 

 warmer air must possess a certain degree of dryness that is not difficult to 

 determine.''^ 



The latter of these two propositions is evident as soon as we allow 

 ^1 to coincide with F as in Fig. 46, and then with F^ still on the ordi- 

 nate TiF'i push sofardownwards that ^0^2 shall come to lie below FF2, 

 a condition which, however, is only to be satisfied so long as T2 does 

 not have too high a value. 



The very paradoxical sentences just set forth lose their extraordinary 

 appearance as soon as we recognize that a mixture of water and un- 

 saturated moist air is not in a condition of equilibrium, but thatiu such 

 a mixture evaporation must always occur unless by some special proc- 

 ess the condition is kept stationary. Such mixtures are exemplified 

 in the fogs and clouds, and during rain. The behavior of such mix- 

 tures has been implicitly investigated in the previous paragraphs and 

 now^ only a few words further need be said. 



