286 ENVIRONMENTAL CONDITIONS. 



offered may be expressed in terms of the water-condensing power of 

 the air, but, as Livingston has remarked, air without actual tendency 

 to precipitate or deposit Uquid or solid water still offers resistance to 

 evaporation : 



"Air without water-vapor offers resistance to evaporation but has no condensing power; 

 it can not deposit water upon a surface, no matter what its pressure may be. The resistance 

 offered by such dry air can be expressed in terms of an equivalent condensing power, however. " 



In answering this second objection to the term "evaporating power 

 of the air," a third possible objection is also partly answered. This 

 objection arises from the various senses in which the word power is 

 used. If we are interested only in the statical phase of the matter 

 before us, then the evaporating power of the air is proportional to the 

 reciprocal of the measure of the capacity of the air-space to retard the 

 vaporization of water, from a liquid or solid water surface exposed to 

 that space. The dynamic phase of the problem of evaporation, how- 

 ever, allows the use of the word "power" in its ordinary physical 

 sense, as denoting the time-rate of doing work. The conditions resi- 

 dent in the air-space are thus thought of as somewhat like a brake on 

 a wheel, and we consider a time-rate of the reciprocal of resistance to 

 evaporation. Thus, our use of the word "power" is not with the 

 meaning of spatial capacity, but we employ the word in its true physical 

 sense, as though the air were a machine acting to retard evaporation. 

 As in other cases of power measurement, it is necessary to measure 

 the power in question in terms of the amount of work capable of being 

 performed in a given time period. Internal conditions, resident in the 

 solid or liquid phase of the water, determine what would be the rate of 

 evaporation if the air-space offered absolutely no resistance, and if 

 these internal conditions remain constant the amount of evaporation 

 occurring per time period is proportional to the reciprocal of the power 

 of the air to behave as though it were condensing water-vapor upon 

 the exposed surface. The reciprocal of the rate at which water would 

 be condensed if all of the tendency of the air-space to retard evapora- 

 tion were effective toward actual condensation is thus proportional to 

 the tendency of the air conditions to allow evaporation to proceed, and 

 this may be relatively measured as a power by determining the amount 

 of water actually vaporized per time period. Of course, the conditions 

 resident within the solid or liquid surface are never even sensibly con- 

 stant for long, and the actual rate of evaporation depends not only 

 upon the evaporating power of the air, as above defined, but also 

 upon the internal conditions. The evaporating power of the air is 

 thus relatively measured as the time rate of the reciprocal of the 

 resistance offered by the air to evaporation, this resistance being 

 measured in terms of equivalent condensation. But condensation is 

 merely negative evaporation, so that when the air conditions are such 

 as to make the external resistance just equal to the internal tendency 



