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HYDRATED SILICIC ACID is determined by boiling 5grms. of the finely ground material with 120c.c. of 5 per cent, sodium carbonate solution for ten minutes. The solution is decanted and the sample again digested; the combined filtrates are acidified and evaporated to dryness, and the silica estimated as usual. A blank may yield as much as Imgm., and should therefore be undertaken. CARBON may be present in clay to the extent of 4 per cent., imparting a dark colour to it. It is estimated by dissolving the clay in hydrofluoric acid in the water-bath and filtering on to a tarred paper. This is best done after heating the clay in the air-bath as high as possible hi order to char organic matter that may not be yet carbonised. VANADIUM, like titanium, is diffused through all granites, and may be present in clay to the extent of 0.1 per cent. It should be reported as the trioxide, as it replaces alumina or ferric iron. It occurs in basic silicate rocks chiefly. MOLYBDENUM, on the other hand, is confined to the more silicious rocks, and is less common than vanadium. For the analysis of these two components the reader should refer to Hillebrand. LIME, gypsum, magnesia, barium sulphate, and pyrites are detected and estimated by the usual routine of chemical analysis. COLLOIDS in clay are estimated by Ashley by malachite