154 NATURE AND PROPERTIES OF SOILS 



expect that a certain amount of heat of condensation will be 

 apparent when it is resumed. 1 Patten 2 and Bouyoucos 3 offer 

 the following quantitative data concerning this point : 



Table XXVIII 



HEAT EVOLVED BY WETTING SOILS DRIED AT 110° C. 



Soil 



Calories to a 

 Kilo of Dry Soil 



Quartz sand 



Norfolk sand 



Hagerstown loam 



Miami silt loam 



Cecil clay 



Superior clay 



Muck (25% organic matter) 

 Peat 



000 

 347 

 1108 

 1742 

 3376 

 5158 

 6413 

 22185 



87. Determination of the hygroscopic coefficient. — 

 The methods for the determination of the maximum hygro- 

 scopicity of a soil, or, in other words, the hygroscopic coeffi- 

 cient, are simple in outline. The soil, in a thin layer, is ex- 

 posed to an atmosphere of definite humidity under conditions 

 of constant temperature and pressure. Complications arise 

 from the necessity of using a very thin layer of soil, from the 

 difficulty of controlling humidity, and from the tendency of 

 capillary water to form in the soil interstices before the hygro- 

 scopic capacity is satisfied. The question of how long the 

 exposure should take place has not been definitely settled. It 



1 The tremendous heat of wetting is probably due to the latent heat 

 of water, to the attraction that soils have for water and to the condition 

 into which the water is transformed. The heat of condensation is so 

 large as to suggest the probability of a change in the aggregation of 

 the moisture thus absorbed. 



2 Patten, H. E., Heat Transference in Soils; U. S. Dept. Agr., Bur. 

 Soils, Bui. 59, p. 34, 1909. 



3 Bouyoucos, G. J., Relationship between the Unfree Water and the 

 Heat of Wetting of Soils and its Significance; Mich. Agr. Exp. Sta., 

 Tech. Bui. 42, Mar. 1918. 



