FLOCCULATION 325 



arisen an attraction between them : hence the aggregation of 

 the particles into floes after the addition of the acid, etc. 



It was shown that with alkalis the results were substantially 

 the same as with acids and salts, but that the combination of 

 the clay with the alkali did not become complete till excess of 

 alkali was present a very frequent condition in chemical reac- 

 tions so that it required a much greater proportion of alkali 

 than of acid to effect flocculation, with a further result, that 

 where the proportion of alkali present was small, and insufficient 

 to combine in any appreciable quantity with the clay, the latter, 

 finding itself in a liquid for which it possessed a certain amount of 

 attraction, subsided less quickly than it would in pure water, 

 so that a small proportion of alkali produces the reverse of floccu- 

 lation, though it is not deflocculation in the true sense, there being 

 no actual diminution in the size of the particles themselves. 



In practice, however, it should be remarked when, for 

 instance, a field is limed the alkali may result in actual defloc- 

 culation, owing to its neutralising the acid in the soil, and forming 

 insoluble and, therefore, inactive salts; and this result will 

 follow, even if there is no acid present other than carbonic acid, 

 for this latter is a flocculating agent, just as stronger acids are. 

 The ease with which a clay soil may be worked is, as is well 

 known, much affected by the nature of the dressings applied to 

 it : the presence of alkali renders it sticky and unworkable, 

 whereas a dressing of minerals improves the working of it. 



Compounds such as those of clay with acids and salts play a 

 very important role in the chemistry of the soil and of the plant, 

 and their importance is every day becoming more fully recognised. 

 They differ materially from ordinary, more stable, chemical 

 compounds, and seem to occupy the border land between these 

 and mere physical mixtures, being known as adsorption com- 

 pounds. The stabler chemical compounds are formed by the 

 intimate union of atoms or molecules, which become, as it were, 

 fused together into a whole, so that the resulting compound 

 possesses properties quite distinct from those of its components : 

 in adsorption compounds, whole molecules, or masses of molecules, 

 become joined together, the components simply adhering to each 

 other, and each retaining for the most part its own distinctive 

 properties. Such compounds may be very easily decomposed, 

 even by physical means : thus, the acid united with clay may be 

 gradually removed by continued washing with water, though 

 not nearly so easily as could some inert substance, such as cane 

 sugar, which had not combined in any way with the clay ; the 



