The proportion of metal in an ore or of hydrocarbon in a coal can be ascertained without serious difficulty by some means of analysis, but with clay the difficulties are so great that, to some extent at least, they must be regarded as being, for the present, insurmountable. This is in no small measure due to the general recognition of all minerals or rocks which become plastic when kneaded with water as 'clays' without much regard being paid to their composition. Consequently materials of the most diverse nature in other respects are termed clays if they are known to become plastic under certain conditions. There is, in fact, at the present time, no generally accepted definition of clay which distinguishes it from mixtures of clay and sand or other fine mineral particles. The usual geological definitions are so broad as to include many mixtures containing considerably less than half their weight of true clay or they avoid the composition of the material altogether and describe it as a finely divided product of the decomposition of rocks. Many attempts have been made to avoid this unfortunate position, which is alike unsatisfactory to the geologist, the mineralogist and the chemist as well as to the large number of people engaged in the purchase and use of various clays; and, whilst the end sought has not been reached as completely as is desirable, great progress has been made and much has been accomplished during the last twenty years. One of the earliest attempts to ascertain whether there is an essential constituent of all clays was made by Seger (7) who used two methods of separating some of the ingredients of natural clays from the remaining constituents. The first of these methods consists in an application of the investigations of Schulze, Schloesing and Schoene on soils, viz. the removal of the finest particles by elutriation; the second is an extension of the method of Forschammer and Fresenius, viz. the treatment of the material with sulphuric acid. To the product containing the clay when either of these methods is used Seger gave the name _clay substance_, but the material so separated is by no means pure clay. The term clay substance must, therefore, be confined to the crude product containing the clay together with such other impurities as are in the form of extremely small particles or are soluble in sulphuric acid. It has not yet been found possible to isolate pure clay from ordinary clays, so that in investigating the nature of what Seger was endeavouring to produce when he obtained the crude clay substance, indirect methods are necessary.