32 PHYSICAL CONSTITUTION OF SOIL 



state that carbonic acid is an effective precipitant of pure 

 clay. Calcium hydrate they found far more effective than 

 any other calcium salt. Pure clay, when precipitated by 

 lime water, did not apparently carry down lime with it. 

 Monocalcium phosphate was very effective as a precipitant. 

 Sulphate and chloride of ammonium were fair precipitants, as 

 also was chloride of sodium. Nitrate of sodium had very 

 little precipitating power. 



Alkaline substances appear generally to favour the diffusion 

 of clay in water, and to destroy flocculation when it has oc- 

 curred. Ammonia and sodium carbonate possess this property. 



The facts now mentioned as to the coagulation of clay have 

 a wide-reaching application. Schloesing points out that the 

 clearness or turbidity of river water depends essentially on 

 the proportion of lime present. For a stream to be capable 

 of depositing the clay brought into it, the water must contain 

 70-80 mgrams. of lime per litre. Glacier streams are always 

 turbid, owing to the purity of the water. The waters of the 

 Loire and Garonne are turbid, containing respectively 27 and 

 36 mgrams. of lime per litre. The Rhone clarifies slowly, the 

 lime and magnesia amounting to 68 mgrams. per litre. The 

 Seine clarifies quickly, the lime being 104 mgrams. per litre. 

 The immediate precipitation of river mud on contact with sea 

 water, giving rise to the formation of banks, bars, and deltas, 

 is well known ; the principal cause of this precipitation is 

 doubtless the coagulation of the clay on mixing with a strong 

 saline solution. 



In soils the same class of facts may be readily observed. 

 Jn the laboratory, a clay soil may be washed on a filter with 

 weak acid, or with water containing lime, but if distilled 

 water is employed the washings become turbid, and the per- 



