THE PHYSICAL PEOPEETIES OF SOILS 59 



of absorbing gases, and easily condensed vapours like 

 water are reduced to the liquid state. The amount of 

 water soils can absorb in this way, however, depends partly 

 on the degree of saturation of the atmosphere as well as 

 upon the extent of the internal surface. 



Deliquescence. The property known as deliquescence is 

 due to the presence in the soil of certain substances which 

 possess the power of attracting water vapour from the air 

 independently of their physical condition. The water 

 probably combines with the substances to form hydrates, 

 as it is not completely evaporated on exposure to dry air, 

 even at a temperature of 100 C. Calcium chloride and 

 phosphoric acid are examples of highly deliquescent bodies, 

 out common salt, hydrates of iron, alumina and silica and 

 other substances commonly present in soils, exhibit similar 

 properties in different degrees. 



The power pf a soil to absorb water vapour from the 

 air is determined either by drying the soil at 100 C. and 

 then exposing it to a saturated atmosphere, or by first 

 exposing it to a saturated atmosphere and then drying at 

 100 C., but the two methods give different results. The 

 experiments make no distinction between hygroscopic and 

 deliquescent moisture, and are probably not of much value 

 as an indication of the properties of the undried soil. All 

 purely hygroscopic water evaporates on exposure to dry air 

 at ordinary temperatures, but the moisture attracted by 

 deliquescent bodies is at least partly retained by them. It 

 seems unlikely that water which requires heat to expel it 

 from the soil can ever be of much benefit to plants. 



The absorptive power of pure sand is practically nil; 

 the grains are too large for hygroscopic action and it con- 

 tains no deliquescent bodies. Pipeclay, owing to its finer 

 texture, can absorb about 9 per cent, of water, and clay 

 soils, containing a large proportion of deliquescent col- 

 loidal hydrates, much more. 



