32 Introduction. 



212 32 = ISO 8 



without having its temperature materially lowered. The 

 molecular energy, therefore, which the one pound of steam 

 contained was 



180 X 5.37 = 9G6.6 units. 



This large amount of energy in steam explains how it is 

 able to do so much work when acting upon the engine pis- 

 ton and why a burn from steam may be so much more se- 

 vere than that from boiling water. 



44. Evaporation Cools the Soil. We have seen that one 

 pound of steam in condensing into water generates OOO.C 

 heat units, and that the reverse statement is also true, 

 namely, to convert a pound of water into the gaseous state, 

 under the mean atmospheric pressure, requires the absorp- 

 tion by that pound of 966.6 heat units. When one pound 

 of water disappears from a cubic foot of soil by evapora- 

 tion, it carries with it heat enough to lower its tempera- 

 ture, if saturated sand, 32.8 F. ; and if saturated clay 

 loam, 28.8 F. 



To dry saturated sandy soil until it contains one-half 

 of its maximum amount of water requires the evaporation 

 of about 9.5 pounds to the square foot of soil surface when 

 this drying extends to a depth of one foot, while the simi- 

 lar drying of clay loam requires the evaporation of 11.5 

 pounds, and 



11.5 9.6 = 2 Ibs. 



or the amount of evaporation which must take place in the 

 clay loam to bring it to the same degree of dryness as the 

 sandy soil. But to evaporate two pounds of water re- 

 quires 



966.6 X 2 = 1933.2 heat units. 



and this, if withdrawn directly from a cubic foot of satur- 

 ated clay loam, would lower its temperature 57.6 F. 

 Here is one of the chief reasons why a wet soil is cold. 



