The mineralogical determination of those species, the chief constituent of which is Manganese, has been for a long time destitute of that precision at which other species had long arrived, whose chemical constitution was better known. Two years ago I published, in a memoir “On the Crystalline Forms and Properties of the Manganese-Ores,” the most accurate information I could then collect, partly from some works on mineralogy, partly from my own observations. In the general descriptions which I propose giving here for the mineralogical illustration of Dr Turner's account of their chemical properties, I have availed myself of the corrections given in the translation of the same paper in Poggendorff's Annals by Professor Gustavus Rose, who has corrected or verified the angles given, and compared them again with nature; so that the statements have gained a considerable accession of authority. I have added the description of that species, which consists of the anhydrous peroxide of manganese, and which, from the difference of its properties from all the rest, whatever may be the mode of its formation, should be considered as a species of its own.