CLASSIFICATION OF CLAYS. 57 



ity; but knowing the rational analysis of the new 

 clay, it would be easy to add quartz or feldspar so as 

 to bring the mixture back to its normal composition. 



The application of the method of rational analysis 

 to impure clays is not quite as satisfactory, but at the 

 same time not as necessary. In the treatment, the 

 iron, if present as oxide, and lime or magnesia, if car- 

 banotes, are dissolved out with the clay-substance. 

 The silicate minerals are grouped with the feldspar, 

 and the clay thus becomes divided into clay-substance 

 (kaolinite, ferric oxide, lime and magnesia carbon- 

 ates), feldspar or feldspathic detritus; and quartz. 

 If the percentage of ferric oxide and carbonates is 

 high, it is necessary to determine them separately in 

 the ultimate analysis. 



In making a rational analysis, the clay is .'treated 

 with strong sulphuric acid, which decomposes the kao- 

 lin into sulphate of alumina and hydrous silica. The 

 former is soluble in water, while the latter is removed 

 with caustic soda, and we get an insoluble residue con- 

 sisting of quartz and feldspar. In this residue the 

 alumina is determined and the feldspar calculated. 



Another way of conducting the rational analysis, 

 and one which is chiefly applicable when the clay con- 

 tains other minerals besides the kaolin, quartz and 

 feldspar, such as carbonate of lime, ferric oxide, or 

 mica, consists in analysing the insoluble residue and 

 calculating the mineral percentages from this. 



THE CLASSIFICATION OF CLAYS. 



As it is possible to find every gradation from the 

 purest to the most impure clays any classification that 

 is attempted, will necessarily be more or less unsatis- 

 factory. It is of course possible primarily to make 



