86 SOILS AND MANURES 



in a state of infinitely fine division. To say that the 

 colloidal state, at least so far as clay is concerned, marks 

 the ultimate limit of division in solids would probably be 

 nearer the truth. 



The addition of a few drops of lime water to the turbid 

 liquid containing the colloidal clay in suspension causes 

 the suspended particles to coalesce and form aggregates or 

 flocules, which soon settle out, leaving the liquid perfectly 

 clear. Mineral acids, acid salts, and some normal salts, 

 e.g., common salt, have a similar effect. Precipitation 

 does not appear to be due to combination with the reagent, 

 as the latter remains in solution and the precipitate can 

 be redisseminated through pure water. 



Mechanical force causes defloculation or destruction of 

 the aggregates, and is applied in various ways at potteries 

 and brickworks for this purpose. Defloculation can also 

 be effected by puddling and kneading, vigorous agitation 

 with water and by boiling. It is promoted by the presence 

 of alkalis and of alkaline salts. : 



According to one hypothesis, the floculating agents 

 lime, alum, common salt, etc. cause displacement of some 

 of the loosely combined water of hydration, and floculation 

 results merely from the tendency to shrink in volume, 

 which all highly hydrated substances exhibit when water is 

 abstracted. The " sailing out" of soaps, albuminoids and 

 other substances of complex molecular structure, has also 

 been attributed to abstraction of water, and it is possible 

 that the floculation of colloidal clay may be analogous. 



Whatever the cause or mechanism of the process there is 

 no room for doubt as to its great practical importance. 

 In the defloculated or gelatinous condition colloidal clay 

 greatly increases the tenacity and blocks up the pores of the 

 soil, and clay soils cannot be successfully cultivated unless 

 kept in the floculent state. It is for this reason that 

 they must not be " worked " or trampled when wet, and 



