62 RELATIONS OF SOIL TO WATER 



The percentage of water expelled at 200 C. from the vapour- 

 saturated soils amounted in one case to 37-4 per cent, of the 

 weight of the dry soil. 



When a substance condenses water from the atmosphere 

 its temperature rises, the water necessarily evolving heat 

 when passing from the gaseous to the liquid state. Stellwaag 

 (Wollny, Forsch. der Agrikulturphysik, v. 210) determined 

 the rise in temperature of various soil constituents, first dried 

 at 1 05 C. and then placed in a saturated atmosphere at 30 C. 

 The changes in temperature observed give an excellent idea 

 of the relative hygroscopic power of the materials experi- 

 mented with. 



Rise in Temperature in Water-vapour at 30 



Quartz Sand r . ... 0-88 C. 



Calcium Carbonate (precipitated) ... 147 



Kaolin 2.63 



Hydrated Ferric Oxide 9-30 



Peat 12^.25 



The moistening of a dry body with liquid water, and 

 especially the combination of a colloid body with water, also 

 produces a rise in temperature. At I o C. the results obtained 

 were as follows. 



Rise in Temperature when moistened with Liquid Water 



Quartz Sand 0.10 C. 



Calcium Carbonate (precipitated) ... 0-28 



Kaolin 0-83 



Hydrated Ferric Oxide 6.60 



Sachs has taught, and his statement is current in modern 

 textbooks, that plants are able to make use of the hygroscopic 

 water in soils ; and that, in fact, a supply of water- vapour in 

 the air is sufficient to preserve such an amount of water in the 



