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matter of irrigation should be ordinarily made on this basis. 

 Heat decreases this capacity for holding water. The poro- 

 sity of soil though depending mainly on the fineness of 

 its particles, also depends on looseness or fineness of tilth. 

 Loose agricultural soil can hold 59% of water, while the same 

 soil shaken down will hold only 45%, and pressed down 

 only 40%. 



85. Capillarity. The capillary power of soils for draw- 

 ing water up from below depends on their porosity. Clay pos- 

 sesses the greatest capillarity and sand and chalk the least. 

 A column of fine clay wetted from the bottom will become 

 wet to a height of i to 2 yards. Quartz sand similarly 

 wetted becomes wet to a height of only \ yd., and chalky or 

 calcareous soil (i.e. soil made up of particles of pure calcium 

 carbonate) to a still less height. The capillary action of soils 

 in lump is less than that of the same soils'.when finely pow- 

 dered or broken down. This is one of the many reasons 

 why cultivation benefits crops. Capillary action takes 3 or 4 

 days before it reaches its final limit. 



86. Hygroscopic power. All porous bodies have the power 

 of absorbing moisture from the atmosphere. The proportion 

 of moisture absorbed depends (i) upon the surface exposed, 

 and (2) on the nature of the substance. Organic substances, 

 as a rule, are more hygroscopic than mineral substances. 

 Wool, silk and hair are highly hygroscopic. Wool absorbs 

 19 to 20% of moisture from air at the freezing temperature. 

 In buying and selling this must be borne in mind. Silk may 

 contain 9 or 10% of latent moisture over and above the 11% 

 of normal moisture, without one noticing it. In buying 100 

 maunds of silk it is quite possible to throw away Rs. 7,000 or 

 Rs. 8,000 in buying superfluous water. Dry seasons should 

 be chosen for buying these substances. Manuring soils with 

 refuse from wool or silk factories or with hair, increase 

 their absorbent power for moisture. Absorbent power varies 

 very considerably in soils. Coarse quartz-sand absorbs 



