(a) Render any kind of platinum bath acid by hydrochloric acid, unless it is already so, and then plunge cleansed iron into it. The platinum is reduced to a black powder, wash, and calcine to a white heat. Dissolving it in aqua regia reconstitutes the platinum chloride necessary for the preparation of the baths.

(6) Reduce by evaporating the bath to dryness, strongly calcine the residue, then wash upon a filter to remove the soluble salts, and again heat to a white heat. The platinum thus obtained is soluble in aqua regia.

(c) In the face of the present high price of platinum, residues and "washes" of this metal are well worth working up. The old developing-baths for platinotype paper, odd bits of the paper itself, spoilt prints, etc, all may be reworked. Old developers may be treated with saturated solution of iron sulphate - 1 part of the saturated solution to 4 parts of the developer will be about right; the mixture should be heated to boiling in a porcelain dish, and the precipitated metal collected on a filter. Bad prints, clippings, etc, of the paper, should be placed on a tin or iron plate and burned, the ashes collected and digested for some hours at 122°-158° F. with aqua regia, and then the solution diluted with an equal volume of water, and the platinum pre-(ipitated with iron as above, or with ammonium, or soda formate, or any other reducing agent.