In a paper read before the Chemical Society I gave a detailed account of a curious reaction of Manganese salts, and showed that Manganese dioxide resembling pyrolusite could be obtained by the reaction. I have since made many samples, and have obtained nodules so like the natural mineral, that I thought that an account of the method might have some interest for mineralogists. When manganous nitrate is dissolved in strong nitric acid and warmed, and a few chips of potassic chlorate added, there is precipitated in the liquid black oxide of manganese in the form of flat plates which, when very thin, are of a beautiful purple, but when thicker are of a red colour. The whole of the manganese is precipitated in this form, but when the strength of the acid and the temperature are made to vary, the distribution of the plates may be changed.