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Platinotype, which may be considered to be the most artistic of photographic printing processes, may be separated into its three modifications—the hot bath and cold bath, in which a faintly visible image is developed, and the Pizzighelli printing-out paper. The hot bath process, again, may be divided into the black and white and sepia papers. I intend to give you a rough outline of the preparation of the paper and working of these modifications, concluding by demonstrating the hot bath method, and handing around prints by it. Platinotype may almost be styled an iron printing process, for, while no trace of iron or its salts is found in the finished print, certain salts of iron are mixed with the platinum salt, which is platinum combined with two atoms of chlorine (PtCl2), as a means for readily reducing it; this, however, cannot be effected without the presence of neutral oxalate of potash, hence the use of the oxalate bath. There is no platinum in the paper for the cold bath process, it being coated with ferric oxalate mixed with a very small quantity of chloride of mercury—somewhere about one grain to an ounce of ferric oxalate solution. When dry it is ready for exposure, which is about three times less than with silver printing.