TENACITY OF SOIL 23 



of the clay. The remainder of the clay is made up of 

 extremely fine, solid particles. Clay has, in fact, a 

 constitution similar to that of common putty. In the 

 purest natural clays the whole of the constituents 

 have the same general chemical composition (hydrated 

 silicate of aluminium), but in soils the non-colloid con- 

 stituents of the clay may be of very various nature. 

 In brick earth this matter is quartz sand; in marl 

 it is carbonate of calcium. 



The condition of clay soils depends much on whether 

 the clay is coagulated or not. When the clay is un- 

 coagulated the soil is sticky, impervious to water, and 

 cannot be reduced to a fine tilth. When the clay is 

 coagulated the soil has a granular structure, is per- 

 vious to water, and can be reduced to powder. Clay 

 is effectually coagulated by frost, which by removing 

 the water from the colloid cement causes it to shrink. 

 It is also coagulated by lime, and by many salts, 

 and especially by salts of calcium. Colloid clay will 

 remain permanently suspended in distilled water ; it 

 is precipitated on the addition of a small quantity of 

 a calcium salt. An application of chalk or lime to 

 clay soils is well known to be extremely effective in 

 diminishing their tenacity, rendering them pervious 

 to water, and more easy of tillage. 



In cultivated sandy soils humates are often of great 

 value as cementing materials ; these, like true clay, are 

 colloid bodies. Schloesing found that 1 per cent, of 

 humic acid, in the form of calcium humate, was as 

 effective as a cement for sand as the presence of 11 per 

 cent, of a fat clay. Humates, however, lose their 

 cementing properties on drying, while clay does not. 

 The improvement of the texture of sandy soils by tlie 



