As the china clays (kaolins) and ball clays on very careful elutriation all yield a product of the same ultimate composition, viz. 39 per cent. of alumina, 46 per cent. of silica, 13 per cent. of water, and 2 per cent. of other oxides, they are generally regarded as consisting of practically pure clay with a variable amount of impurities. Many years ago Fresenius suggested that these non-clayey constituents of clays should be calculated into the minerals to which they appeared likely to correspond so as to obtain a result similar to that obtained by Seger without the disadvantages of the treatment with sulphuric acid and as supplementary to such treatment in the case of red-burning and some other clays. More recent investigators have found that if a careful microscopic examination of the clay is made the results of estimating the composition from the proportion of the different minerals recognizable under the microscope and by calculation from the analysis of the material agree very closely and are, as Bischof (28) and, more recently, Mellor have pointed out, more reliable than the 'rational analysis' in the case of impure clays. If care is taken to make a microscopical examination identifying the chief impurities present the calculation from the analysis may usually be accepted as sufficiently accurate, but it is very unsatisfactory to assume, as some chemists do, that the alkalies and lime in the clay are all in the form of felspar and that the silica remaining in excess of that required to combine with the alkalies, lime and alumina is free quartz. Some clays are almost destitute of felspar but comparatively rich in mica, whilst others are the reverse, so that some means of identifying the extraneous minerals is essential. When this is not used, the curious result is obtained that German chemists calculate the alkalies, etc. to felspar whilst the French chemists, following Vogt, calculate them to mica; English ceramic chemists appear undecided as to which course to follow, and some of them occasionally report notable amounts of felspar in clays quite destitute of this mineral! A statement of the composition of a 'clay' based on a mechanical separation of the coarser ingredients followed by an analysis of the finer ones and a calculation of the probable constituents of the latter, as already described, is known as a _proximate analysis_ in order to distinguish it from an _ultimate analysis_ which states the composition of the whole material in terms of its ultimate oxides. A proximate analysis therefore shows the various materials entering into the composition of the clay in the following or similar terms: