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In part IV a preliminary account was given of the discovery that the benzene-pressure-extracted "residue" of a typical bituminous coal can readliy be oxidized by means of an alkaline solution of potassium permanganate with formation of considerable quantities of benzenoid acids. Four years later, in Part VI, the quantitative results of such oxidations of the "residues" derived from five selected brown coals, lignites, and bituminous coals were detailed. It was shown that each "residue," which constituted upwards of 85% of the corresponding total coal substance, can be so oxidized to a mixture of carbonic anhydride, acetic, oxalic and benzenecarboxylic acids. Thus, for example, the carbon of such "residue" from a typical Durham coking coal was distributed among the oxidation products as follows:— 42·4% as carbonic anhydride 1·7% as acetic acid 6·5% as oxalic acid 48·8% benzenecarboxylic acids. Moreover, from the crude mixtures of benzene carboxylic acids so obtained, whose mean compositions closely approximated to that of a benzene-tricarboxylic acid, were isolated phthalic, isophthalic, terephthalic, trimellitic, mellophanic, pyro-mellitic, benzenepentacarboxylic and mellitic acids. It was also shown that the oxidation of the coal substance to such benzenoid acids proceeds through the successive formations of colloidal humic acids, and probably also of intermediate crystalline acids.

Researches on the chemistry of coal VIII—The development of benzenoid constitution in the lignin-peat-coal-series