This stone is the ponderous spar, or native sulphate of barytes, the phosphoric property of which was first discovered by an Italian shoemaker. If it be first heated to ignition, then finely powdered, and made into a paste with mucilage, and this paste divided into pieces a quarter of an inch thick, and dried in a moderate heat, be exposed to the heat of a wind furnace, by placing them loose in the midst of the charcoal, a Pyrophorus will be obtained, which, after a few minutes' exposure to the sun's rays, will give light enough in the dark to render the figures on the dial-plate of a watch visible.