64 THE PHYSICAL PROPERTIES OF SOILS 



TWELFTH EXERCISE 



THE ABSORPTIVE POWER OF SOILS 



In common with other finely divided substances, 

 soils have the power of absorbing water vapor and 

 other gases from the atmosphere. The hygroscopic 

 capacity of different soils may be compared by exposing 

 them to a saturated atmosphere for a given length of 

 time under standard temperature conditions, and ob- 

 serving the increase in weight. 



METHOD OF DETERMINATION 



Weigh out carefully into shallow aluminum trays 

 or pans, 25-gram samples of each of the following 

 soils : 



Tray No. i — Sand. 



Tray No. 2 — Loam or silt. 



Tray No. 3 — Clay. 



Tray No. 4 — Mixture, one part sand, one 



part organic matter. 

 Tray No. 5 — Mixture, one part loam, one 



part organic matter. 



Spread thin in the trays, dry to constant weight, 

 either in a drying oven or over sulphuric acid, 

 cool in a desiccator, and place the trays, with lids off, 

 into a constant temperature chamber in which the air 

 is kept saturated with water vapor. Allow the samples 

 to remain in contact with the saturated atmosphere 

 until a constant weight is obtained. An empty tray 

 should be kept with those containing the soil, and its 



