Owing to the exceptional minuteness of its particles, it is extremely difficult to ascertain whether pure china clay or kaolin is crystalline or amorphous. Johnson and Blake (21) found that all the specimens they examined 'consisted largely of hexagonal plates' and that in most kaolins 'these plates are abundant--evidently constituting the bulk of the substance.' This observation is contrary to the experience of most investigators, the majority of whom have found the bulk of the material to be amorphous and sponge-like, but a small portion of it to consist of hexagonal or rhombic crystals. Mellor (22) has proposed the name _clayite_ for this amorphous material, the crystalline portion being termed _kaolinite_ as suggested by Johnson and Blake. Both kaolinite (crystalline) and clayite (amorphous) yield the same results on analysis and correspond very closely to the formula H4Al2Si2O9 or Al2O3·2SiO2·2H2O, so that it is most probable that they are the same substance in different physical states. According to Hickling (36) the general impression that the particles of china clay are amorphous is due to the use of microscopes of insufficient power. With an improved instrument, Hickling claims to have identified the 'amorphous' portion of china clay with crystalline kaolinite, the clay particles (fig. 17) being in the form of irregular, curved, hexagonal prisms or in isolated plates. The former show strong transverse cleavages. The index of refraction and that of double refraction agree with those of Anglesea kaolinite crystals, as does the specific gravity. In spite of their great purity, commercial china clays and kaolins are almost devoid of plasticity, nor can this property be greatly increased by any artificial treatment. This has led to the conclusion that plasticity is not an essential characteristic of the clayite or kaolinite molecules, but is due to physical causes not shown by any investigation of the chemical composition of the material. In addition to the specially purified kaolins just described, alkaline kaolins, siliceous kaolins and ferruginous kaolins are obtained from less pure rocks and do not undergo so thorough a treatment with water. Some of these varieties are not improbably derived from transported kaolins, as they occur in Tertiary strata, and so bear some resemblance to the white fireclays on the Carboniferous limestone of Staffordshire, Derbyshire and North Wales, though the latter are far more plastic. To be of value, a china clay or kaolin must be as white as possible and must be free from more than an insignificant percentage of metallic oxides which will produce a colour when the clay is heated to bright redness. If the material is to be used in the manufacture of paper, paint or ultra-marine, these colour-producing oxides are of less importance providing that the clay is sufficiently white in its commercial state.