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been so often assumed that there is a widespread impression that it 

 really exists as a definite chemieal compound, though it has never 

 been satisfactorily isolated. A further objection to this belief in 

 the existence of a single substance as the essential ingredient of all 

 clays, is the fact that the composition and properties — especially 

 the plasticity — of the elutriated product differs with the origin and 

 nature of the " clay " from which it is obtained. A further objection 

 is that all attempts to obtain a pure product by chemical means or 

 to produce a synthetic clay have failed. 



It has also been suggested by W. and D. Asch^ that the essential 

 ingredient of all clays is not to be expressed by a single substance, 

 but by a large number of substances, each of which have a general 

 similarity in composition, but differing from each other in the 

 arrangement of the atoms in highly complex molecules. Thus, all 

 clays possess properties corresponding to those of a series of insoluble 

 acids and may, therefore, be regarded as alumino-silicic acids. 

 W. and D. Asch- go stiU further and suggest that the essential 

 substances in aU clays are alumino-silicic acids, the atoms of which 

 are arranged to form several ring- compounds united together, each 

 ring containing six atoms of either silicon or aluminum together 

 with the requisite number of oxygen and hydrogen atoms (the latter 

 being in the form of hydroxyl groups) to form a saturated compound. 

 In most cases, tAvo or more hydrogen atoms are assumed to be replaced 

 by those of sodium, potassium, calcium, magnesium, or iron. 



Asch's theory has been worked out in great detail with regard to 

 the available evidence, but for *ts ultimate proof it requires the 

 synthetic production of clays of various compositions and this 

 synthetical proof has not yet been accomplished. The synthesis of 

 silica in the form of a hexagonal ring compound, SisOia, by G. Martin^ 

 in 1913 lends some support to the suggested constitution of the purer 

 clays. It has also been suggested that plastic clays are compounds 

 of " alumma " and '' silica " with organic groups. This theory does 

 not appear to have been very fully investigated. It has the dis- 

 advantage of bemg largely inapplicable to the purer kaolins which 

 are almost devoid of organic matter. 



It was, at one time, thought that the plasticity of clays is due to 

 the presence of bacteria and bacterial products, but this has not been 

 proved, and appears to be improbable. 



Although there is a considerable amount of evidence of the existence 

 of one or more definite chemical compounds which are the essential 

 ingredients of clays and, therefore, to be regarded as " true clay," 

 there are numerous properties of clays which cannot be explained 

 by any purely " chemical " theory, and of these the most important 

 is the plasticity. There are also properties which are capable of other 

 explanations, particularly those based on the coUoidal nature of 

 clays. 



The existence of colloidal matter in clays was first estabUshed 

 by Th. Schloesing* in 1872, but the most systematic exposition on 

 the colloidal properties of clays is that of P. Rohland^ in 1891, and 

 more recently the possession of coUoidal properties of clays has been 

 generally recognised*. 



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