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* rational analysis.' But it was Seger, in the later 'seventies, who developed the method, although it is still far from per- fect. The idea of the process is to decompose the clay with sulphuric acid, which is supposed to leave the spar and quartz untouched. The residue is boiled with alkali to remove the liberated silica, and this residue is then fused with carbonates, and the alumina in it estimated in the usual way, or else treated by the Lawrence Smith method for estimating alkalies. The felspar is calculated from either the alumina or alkalies just found. Felspar (ortho- clase) has the theoretical percentage composition : SiO 2 , 64.63 ; Al a O 8 , 18.49 ; K^O, 16.88 the alumina being 1.085, and the silica 3.828 times the alkali, and the silica-alumina ratio is 3.5 : 1. Clay contains theoretically 46.33 SiO 2 ; 39.77 A1 3 O 3 ; and 13.9 H,O per cent. the silica-alumina ratio being 1.17: 1 Having found the clay and spar, the quartz is given by difference. Rational analysis, however, has several drawbacks. Mica was neglected by Seger, who returned his results as clay, felspar, and quartz. The modern French school goes too far in the other direction, and returns mica to the ex- clusion of spar. There can be little doubt that a seemingly 22 CERAMIC CHEMISTRY. stable bed of clay contains clay, spar, mica, quartz, rutile, hornblende, and many other minerals, all reacting