Stones per cent. Gravel " Coarse sand " Medium sand " Fine sand " Silt " Felspar or mica dust " Silica dust " 'True clay[12]' " Moisture " Carbon " Other volatile matter " [Footnote 12: In analytical reports a note should be appended stating that the figure under this term shows the proportion of the nearest approximation to true clay at present attainable.] For some purposes it is necessary to show the proportion of calcium, iron and other compounds as in an ordinary ultimate analysis. A comparison of the foregoing with an ultimate or 'ordinary' analysis of a clay (p. 16) will show at once the advantage of the former in increasing our knowledge of the essential constituent of all clays, if such a substance really exists. Its absolute existence is by no means proved, for, as will have been noticed, its composition is largely based on assumption even in the most thorough investigations, particularly of the admittedly less pure clays. In the purer clays the problem is much simpler and in their case an answer of at least approximate accuracy can be given to the question 'What is clay?' Even with these purer clays it is not sufficient to study an analysis showing the total amount of the silica, alumina and other oxides present; it is still necessary to effect some kind of separation into the various minerals of which they are composed. When, however, the accessory minerals do not exceed 5 per cent. of the total ingredients their influence is less important and the nature and characteristics of the 'clay substance' itself can be more accurately studied. By careful treatment of well selected china clays, for example, it is possible to obtain a material corresponding to the formula Al2O3·2SiO2·2H2O within a total error of 1 per cent., the small amount of impurity being, as far as can be ascertained, composed of mica. So pure a specimen of clay is found on microscopical examination to consist of minute irregular grains of no definite form, together with a few crystals of the same composition and identifiable as the mineral 'kaolinite' (p. 107). This 'amorphous' material, which appears to be the chief constituent of all china clays and kaolins, has been termed _clayite_ by Mellor (22). Johnson and Blake, Aron and other observers have stated that the majority of the particles in china clays and kaolins are crystalline in form. Owing to their extreme smallness it is exceedingly difficult to prove that they are not so, though for all ordinary purposes they may be regarded as amorphous, the proportion of obviously crystalline matter present in British china clay of the highest qualities being so small as to be negligible.