SHRINKAGE ON DRYING 35 



Frost is an effective agent in bringing about the shrinkage 

 of colloid clay. If water containing colloid clay in suspension 

 is slowly frozen, the water separates as clear ice, and the 

 clay is concentrated in the unfrozen liquid, or finally separated 

 as a solid film. The same action takes place in a soil when 

 it freezes ; the drainage waters obtained during a thaw are 

 always remarkable for their brightness. 



Shrinkage on Drying. The different constituents of soil 

 behave very differently on drying. It is a well-known 

 fact in the arts that wet sand suffers little change of 

 volume when dried ; sand is thus always employed when 

 making moulds for casting metals. The clay used by 

 the potter is equally well known to undergo a considerable 

 contraction on drying ; all articles of pottery have therefore 

 to be made considerably larger than the size finally in- 

 tended. 



Schiibler carefully determined the amount of contraction 

 suffered by various soil constituents during drying. Cubes 

 made of wet siliceous or calcareous sand did not change in 

 volume on drying. The purest clay available, when treated 

 in the same way, showed a contraction of 18-3 per cent, of 

 its volume ; a sandy clay showed a contraction of 6 per cent. 

 The greatest contraction on drying was exhibited by humus, 

 the amount being 20 per cent, of its volume; the humus 

 employed was apparently obtained from the centre of a 

 decayed tree. The various soils he tried varied in their rate 

 of contraction according to the proportions of humus and 

 clay which they contained. An arable soil shrank to the 

 extent of 12 per cent., and a garden soil 14-9 per cent, on 

 drying. From these results we see that contraction on drying 

 is largely determined by the presence of colloid matter in 



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