RELATIONS OF SOILS TO HEAT. 307 



Influence of Evaporation. In treating' of the Conserv- 

 ation of Soil Moisture (chapter 13), the effects, conditions and 

 control of evaporation from the soil have already been dis- 

 cussed from several points of view ; so that a summary review 

 of the subject must suffice in this place. 



It has been stated above that in the case of an average loam 

 soil saturated with water, the heat required to raise the tem- 

 perature of the water one degree would be about twice that 

 needed to so change the dry soil material itself. But if it is 

 required to evaporate the same amount of water from the soil, 

 nearly ten (9.667) times that amount of heat will be required; 

 or in the case assumed, twenty times as much as would suffice 

 to raise the temperature of the dry soil through an equal in- 

 terval of temperature. While in a few cases the cooling of 

 the soil by evaporation is desirable, in the vast majority of 

 cases it is injurious to the progress of vegetation, and should 

 be restricted as much as possible by the means outlined in a 

 former chapter. 



Formation of Dew. There is, however, another aspect of 

 evaporation from the soil which has been long misunder- 

 stood, although the true state of the case was partially recog- 

 nized long- ago. Dew is in common parlance said to " fall," 

 it being supposed that, like rain, it is derived from the atmos- 

 phere. While this is partially true, inasmuch as from very 

 moist, and notably from foggy air dew is frequently deposited 

 on grass and foliage generally, as well as on wood and other 

 strongly heat-radiating surfaces; yet as a matter of fact, in 

 by far the majority of cases, as shown by II. K. Stockbridge l 

 and confirmed by everyday observation, dew is formed from 

 the vapor rising from the warmer soil into a colder atmos- 

 phere, and condensed on the most strongly heat-radiating sur- 

 faces near the ground, such as grass, leaves both green and 

 dry, wood, and other objects first encountering the rising 

 vapor. In manifest proof of this it will be noted that very 

 heavy dews may be seen on the ground, when the roofs of 

 houses as well as the higher shrubs and trees remain perfectly 

 dry. In winter this may be most strikingly seen in the 

 deposition of hoar-frost in and immediately around the cracks 

 of plank sidewalks, whose surface remains dry. 



1 " Rocks and Soils," pp. 175-189. 



