Crystallised, substance from a blast-furnace at Mostyn, in N. Wales, having been brought under my notice by Mr. Heywood, of St. Mellons, it was thought worth while to subject it to a careful examination. The largest of the crystals was about 6 mm. long and 2 mm. in diameter. They were translucent, varying in eolour from almost colourless to honey-yellow, and were arranged in tufts on what seemed to be altered firebrick. They yielded a pale yellow powder of which the specific gravity was found to be 5·605.