THE PHYSICAL PEOPEETIES OF SOILS 85 



Tenacity. In the moist condition the particles of soil 

 exhibit a certain tendency to cohere together owing to the 

 surface tension of the water. When the particles are very 

 small, as in clay, the cohesive tendency is greatly inten- 

 sified and produces that quality of " tenacity " which 

 makes clay soils " heavy " to cultivate. The tenacity dimi- 

 nishes as the size of the particles increases, and ultimately 

 becomes imperceptible. " A rope of sand " is a proverbial 

 expression to indicate lack of cohesion. Tenacity is greatly 

 increased by the presence of gelatinous hydrates and other 

 colloidal substances which colligate the particles. In the 

 absence of such cementing agents the particles show but 

 little tendency to cohere when dry. The most important 

 cementing agents are colloidal clay, humic acid, and the 

 gelatinous hydrates of iron, alumina and silica. 



It is well known that when a sample of strong clay is 

 thoroughly disintegrated and shaken up with water the 

 bulk of it settles to the bottom of the vessel on standing 

 for a few hours but the water remains turbid for an inde- 

 finite period. The solid matter which produces this tur- 

 bidity is called colloidal or coagulable clay. It was at 

 one time supposed that the chemical composition of this 

 substance was essentially different from that of the par- 

 ticles which remained suspended only for a few hours. 

 There is some reason to believe that it may be perhaps more 

 highly hydrated. This, however, is uncertain, and it is 

 evident that it does not differ much in other respects. 

 The extremely minute size of the particles of colloidal clay 

 is sufficient to account for its cementing power and most 

 of its other properties. 



The view has been expressed that the permanent sus- 

 pension of colloidal clay marks only a lower limit of solu- 

 tion. The phrase is ambiguous, and may be equally applied 

 to all other colloid or gelatinous substances which, when 

 they appear to be dissolved, are probably merely suspended 



