By regarding the aluminium as a nucleus, as above, and some aluminium silicates as hypothetical alumino-silicic acids, as suggested by Ulffers, Scharizer, Morozewicz (29) and others, clay substance may be conveniently considered, along with analogous substances, as forming a special group quite distinct from the ordinary silicates. In this way Mellor and Holdcroft (29) consider that clay substance is not a hydrated aluminium silicate--as is usually stated in the text-books--but an alumino-silicic acid, the salts of which are the zeolites and related compounds. From this hypothesis it naturally follows that clay substance is analogous to colloidal silica which has been formed by the decomposition of a silicate by means of water and an acid. If this view be correct, pure clay substance or true clay is a tetra-basic alumino-silicic acid H4Al2SiO9 or Al2Si2O5(0H4). That its acid properties are not readily recognizable at ordinary temperatures is due to its inertness; at higher temperatures its power of combination with lime, soda potash and other bases is well recognized, though the reactions which occur are often complicated by decompositions and molecular re-arrangements which occur in consequence of the elevated temperature. There are a number of minerals which closely resemble clayite or pure clay substance in composition, the chief difference being in the proportion of water they evolve on being heated. Thus _Rectorite_ H2Al2Si2O8, _Kaolinite_ H4Al2Si2O9, _Halloysite_ H6Al2Si2O10 and _Newtonite_ H10Al2Si2O12. In the crystalline form these minerals may be distinguished from each other by means of the microscope, but as the chief materials of which clays are composed appears to be amorphous it is impossible to ascertain with certainty whether a given specimen of clay substance is composed of a mixture of these analogous minerals in an amorphous form or whether it consists entirely of clayite, _i.e._ the clay substance obtained from china clay. As already stated, the thermal reactions which occur on heating clayite appear to be characteristic of kaolinite whilst halloysite is completely decomposed at a temperature somewhat below 200° C.; but the not improbable presence of two or more of these alumino-silicic acids in clays of secondary or multary origin makes it almost impossible to determine whether clayite is an essential constituent of all clays or whether the purest clay substance (pelinite) obtained from some of the more plastic clays does not possess a different chemical composition as well as different physical properties.