HEAT OR CALORIC. 109 



needs much less moisture and there radiation is much less ener- 

 getic.* 



It has been already mentioned that a principal cause of the per- 

 manency of snow on high mountains, is the diminution of capacity 

 for heat in the air, in consequence of its rarefaction ; it rises often, 

 highly charged with aqueous vapor, which the cold precipitates 

 abundantly. 



(i.) Circumstances which influence evaporation. 



Surface'. As natural evaporation proceeds from the surface only, 

 the more extensive the surface, other things being equal, the more 

 rapid is the evaporation. 



Water in a bottle, with a narrow open mouth, will waste away very 

 slowly, but the same quantity of water, in a wide and shallow basin, 

 will evaporate much more rapidly. In a narrow-mouthed vessel, also 

 the pressure of the vapor which is formed, will react to retard the evap- 

 oration. Agitation promotes evaporation by enlarging the surface, 

 and by exposing warmer particles successively. 



Temperature. The effect of increased temperature on evapora- 

 tion, is very familiar ; hot fluids evaporate more rapidly than cold 

 ones, in proportion as their temperature is higher. 



Vapor in the air. As a given temperature can raise only a given 

 quantity of vapor into the air, it follows that evaporation will be more 

 or less rapid, according as the quantity of vapor already in the air, 

 is more or less considerable. In a very dry air, the evaporation is 

 always more rapid than in a moist air, and when the vapor already 

 in the atmosphere, is the maximum, that the given temperature can 

 sustain, there will be no evaporation. 



Pressure. The principles that have been established under the 

 head of vapor, are applicable here. Evaporation is more or less 

 rapid, as the pressure is greater or less. Atmospheric pressure re- 

 tards evaporation ; hence, it is remarkably accelerated in the vacu- 

 um of the air pump ; but the same quantity of vapor is raised in the 

 end, whether the atmosphere be present or not ; the only difference 

 is in the rapidity of the process. "Mr. Dalton found that the tension 

 or elasticity of vapor, is always the same, however much the press- 

 ure may vary, so long as the temperature remains constant, and liquid 

 enough is present for preserving the state of saturation, proper to the 

 temperature. If, for example, in a vessel containing a liquid, the 

 space occupied by its vapor, should suddenly dilate, the vapor it con- 

 tains will dilate also, and consequently suffer a diminution of elastic 

 force ; but its tension will be quickly restored, because the liquid 

 yields an additional quantity of vapor, proportional to the increase 

 of space. Again, if the space be diminished, the temperature re- 



* For a description of Mr. Leslie's ^Ethrioscope, See Murrays' Elements 6th Ed- 

 Vol. I. pa. 199. 



