The 'clays' from which the so-called 'natural' or 'Roman cements' are made by simple calcination and crushing, usually fuse at a lower temperature than do the mixtures used for Portland cement, and unless their composition is accurately adjusted they yield a product of such variable quality as to be unsuitable for high class work. =Fuller's earth= is a term used to indicate any earthy material which can be employed for fulling or degreasing wool and bleaching oil. True fuller's earth is obtained chiefly from the neighbourhood of Reigate, Surrey, Woburn Sands, Bedfordshire and from below the Oolite formation near Bath, but owing to the scarcity of the material and the irregularity of its behaviour, china clay is now largely used for the same purpose. True fuller's earth is much more fusible than the white clays usually substituted for it, and when mixed with water it does not form a plastic paste but falls to powder. As the chief requirement of the fuller is the grease-absorbing power of the material there is no objection to the substitution of other earths of equal efficiency. Fuller's earth does not appear to be a true clay, though its constitution and mineralogical composition are by no means clearly known. T. J. Porter considers that it is chiefly composed of montmorillonite (Al2O3·4SiO2H2O), anauxite, (2Al2O3·9SiO2·6H2O), and chalk with some colloidal silica and a little quartz. It therefore appears to resemble the less pure kaolins, but to contain little or no true clay, though in many respects it behaves in a manner similar to a kaolin of unusually low plasticity. =Other clays= of commercial importance, with further details of the ones just mentioned, are described in the author's _British Clays, Shales and Sands_ (2). CHAPTER VI CLAY SUBSTANCE: THEORETICAL AND ACTUAL Having indicated the origin, modes of accumulation and general characteristics of the numerous materials known as 'clay,' it now remains to ascertain what substance, if any, is contained in all of them and may be regarded as their essential constituent, to which their properties are largely due. Just as the value of an ore is dependent to a very large extent on the proportion of the desired metal which it contains, and just as coal is largely, though not entirely, esteemed in proportion to the percentage of carbon and hydrogen in it, so there may be an essential substance in clays to which they owe the most important of their characteristics.