192 § lOT. ANALYSIS OF SOILS AND EOCKS. 



a. Relation of the Soil to Vapor of Water. — 1, 

 Power of retaining liygroscopic water in its pores. — This 

 is measured to some extent by the determination of hy- 

 groscoi3ic moisture (§ 98). The proportion of humus 

 remaining about the same, the power of the soil to retain 

 moisture is very closely related to the amount of clay it 

 contains, while this j)ower is greatly increased by an 

 increased proportion of humus. 



2. It may be interesting to observe the relation Of ttlis 

 property of tlie air-dried soil to the temperature.— 

 For this j^urpose, spread a layer of soil, accurately weigh- 

 ed, about 3 mm. thick over the bottom of a shallow zinc 

 tray, and note the changes in w^eight from day to day, 

 when it is exposed to direct sunlight while protected 

 from currents of air, or when exposed to a temperature 

 of 20°, 30% and 40° C. 



Also, expose the soil to an atmosphere that is saturated 

 with moisture^ by putting it in the same shallow tray to- 

 gether with a shallow vessel of water, under a bell-jar, 

 and weighing it three or four times every 24 hours. An 

 empty tray of the same size should be put under the same 

 bell-jar, and any changes in the weight of this deducted 

 from the differences in the weight of the other. 



Sandy soils and loams usually become nearly saturated 

 in an experiment like this, in the first 24 hours, and 

 change but little in weight thereafter. 



The quantity of Avater absorbed varies of course with 

 the temperature, and with the kind of soil ; but tlicse 

 variations are confined within narrower limits than when 

 the soils are exposed to the air under ordinary circum- 

 stances. The amount of water absorbed from this satu- 

 rated atmosphere ranges between 0.2 and 2.5°!^ of the 

 weight of the completely dry soil. 



The same soil, in its tray, may he exposed to the night 

 air^ to determine the amount of water that it will con- 

 dense from the atmospliere imder these circumstances ; 



