THE SOIL. 15 



dency to gather into large clusters or lumps than in 

 the case of the sandy soils. The action of the surface 

 tension of the water, however, is not the only thing 

 which produces this effect. Some salts in solution in 

 this water have the effect of acting as a cement between 

 the soil grains, so that as the water dries out the soil 

 grains are held firmly together often in very hard lumps. 

 Cause of Shrinkage of Soils. The tendency to 

 shrink possessed by the soils as they dry out is because 

 the films of water are at first quite thick, but become 

 thinner as moisture is lost by evaporation and as con- 

 traction results a tension is produced which is finally 

 relieved by cracking at the weakest point causing the 

 development of checks and cracks running all through 

 the soil. If a tin can with sharp edges is forced down 

 into the soil and then carefully dug up and turned into 

 an upright position and examined by filling carefully 

 with water, it will be found that from a third to a half 

 of the can was occupied by air alone, indicating that the 

 soil only occupies from one-third to two-thirds of the 

 space. This tendency to form soil aggregates or 

 crumbs is illustrated in Fig. 6. 



FIG. 6. 



