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with each other according to partially reversible equations, though the general tendency is towards loss of alkalies and formation of hydrated aluminium silicates such as kaolinite, halloysite, pholerite, and the simplest micas. Further, the assumptions that sulphuric acid attacks only the clay and that spar is unaffected by alkalies are unjustified. Many micas are decomposed by the acid, and Brockmann cites two cases where 18.6 and 16.1 per cent of orthoclase were also dissolved. This was supported by Langenbeck, who states that as much as 20 per cent, of the felspar present may be dissolved with the clay. But Seger, and also Jackson and Rich, found that felspar is decomposed by acid to the extent of only 2.24 per cent., and Jackson and Rich found that the subsequent treat- ment with caustic soda also attacked the felspar, no end point being reached. This, they think, may account for the high results of Langenbeck ; but, nevertheless, the general opinion of modern chemists is undoubtedly against the rational method. The behaviour of micas under the treatment is still a matter of uncertainty. Dana states that biotite and phlogopite are completely, and glauconite, lepidolite, and muscovite slightly, decomposed by acid. He also says that plagioclase felspars are more attacked than orthoclase. And there is no doubt that minerals richer in alkali than felspar, such as nepheline and hatiyne, are completely decomposed. It is obvious, then, that the " rational method " has