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At the present time, the most probable theories of the constitution 

 of clays are as follows : — 



(a) Clays are a mixture of adventitious minerals (such as 

 sand) and one or more alumino-silicic acids, the latter being 

 the true clays. 



(6) Clays are adventitious mixtures of an inorganic colloidal 

 compound, or of several analogous compounds, and of inert 

 minerals such as sand. 



(c) Clays are mixtures of alumina and sihca or other simple 

 mutually precipitated colloids with other non-colloidal minerals, 

 such as sand. 



A little consideration will show that (a) is not necessarily incom- 

 patible with either (6) or (c), as the two latter do not give any indication 

 of the chemical composition of the coUoidal matter. Moreover, (c) is 

 not applicable to aU clays, though it may be to some, so that present- 

 day views of the constitution of clays may be reduced to regarding 

 them as mixtures of non-clayey material (sand, &c.), with either 

 (1) a complex compound possessing colloidal properties, or (2) a 

 mixture of coUoidal sihca and alumina. 



There is evidence in support of both these theories and no 

 comprehensive combination of both of them has been pubUshed, yet 

 neither theory alone explains all the facts, unless it is sufficient to 

 regard the first theory as applying to some clays, whilst the second 

 is more applicable to others. , 



It appears quite certain that commercially useful clays are not 

 whoUy coUoidal in character ; they rather resemble a mass of mineral 

 particles, each covered with a film of colloidal matter. If the latter 

 could be wholly separated, it would not possess all the properties 

 which make a clay technically useful, and in this respect the application 

 of the term clay to mixtures of sand and colloidal matter would appear 

 to be justified. The laterite clays which are mdely distributed in 

 the tropics, are characterised by a large proportion of alumina and 

 sUica soluble in hydrochloric acid. The ratio of these two oxides is 

 very variable, and seldom reaches 1 : 2 which is a conspicuous feature 

 of the purer British clays. This great variation makes it more 

 probable that the laterite clays are merely mixtures of coUoidal sihca 

 and alumina ; their other properties resemble those of such a mixture 

 rather than those of typical clays, and the conclusions based on the 

 results of elutriation may require to be received with caution. 



The proportion of coUoidal matter which can be definitely 

 separated from clays is extremely small, being less than 3 -per cent.* 

 in the "most highly plastic specimens.* Many investigators find it 

 difficult to beheve that so smaU a proportion can account for such 

 great differences in the behaviour of lean and highly plastic clays, 

 and have urged this as an argument against the plasticity of clays 

 bemg due to the coUoidal material present. On the other hand, 



* Ashley' has proposed to determine the relative amounts of colloids in clays 

 by observing the amount of each required to just decolourise a standard solution 

 of malachite green. This method, whilst useful for comparative purposes, 

 gives no idea of the absolute amount of colloidal matter present. 



