In this communication Mr. Faraday shows that when sulphuric acid and naphthaline act upon each other, a peculiar compound possessed of distinct acid characters is the result. This acid is most readily obtained by heating two parts of naphthaline with one of sulphuric acid. The mixture concretes on cooling, and separates into two parts, the uppermost of which is little else than naphthaline, but the lower, heavier part contains the peculiar acid, which, being soluble in water, is easily separated by that fluid, not, however, pure, but still containing mixed sulphuric acid. The author, however, obtained the pure acid by decomposing its compound with baryta, which is soluble, by sulphuric acid. It then had a bitter sour taste, and formed a distinct class of salts with the different bases, all of which are soluble in water and in alcohol, and combustible. By careful evaporation of the aqueous solution of this acid, a white crystalline deliquescent solution was obtained, evolving water when heated, and at high temperatures affording sulphurous acid, charcoal, and naphthaline. To determine the ultimate component parts of this acid, its compound with baryta was subjected to rigid analysis.