Copper, iron, and zinc can be rapidly determined in such a solution, as follows. Dilute with water to 10 or 15 c.c., add an excess of ammonia, and filter. The precipitate will contain the iron as ferric hydrate; dissolve it in a little hot dilute sulphuric acid: reduce with sulphuretted hydrogen; boil off the excess of gas, cool and titrate with standard potassium permanganate (p. 236). Determine the copper in the filtrate colorimetrically (p. 203); but avoid further dilution. Then add dilute hydrochloric acid, so as to have an excess of 4 or 5 c.c. after neutralising the ammonia; add some clean strips of lead foil, and boil until the solution has for some time become colourless. Titrate with standard potassium ferrocyanide (p. 263) without further dilution, and bearing in mind that at most only one or two c.c. will be required. ~Examination of an ore for "cyanicides."~--Place 100 grams of the ore with 200 c.c. of a cyanide solution of known strength (say .1 or .2 per cent.) in a bottle and agitate for a definite time, such as one or two days. Filter off some of the liquor and assay for cyanide, using say 20 c.c. Calculate how much cyanide has been destroyed in the operation. Evaporate 20 c.c. with sulphuric or nitric acid and examine for metal. Test another portion for sulphides, &c. The student who has mastered the methods of assaying can greatly improve himself by working out such problems as the above. PLATINUM. Platinum occurs in nature in alluvial deposits associated with gold and some rare metals, generally in fine metallic grains, and, occasionally, in nuggets. It is a grey metal with a high specific gravity, 21.5 when pure and about 18.0 in native specimens. It is fusible only at the highest temperature, and is not acted on by acids. It is dissolved by warm aqua regia, forming a solution of "platinic chloride," H_{2}PtCl_{6}. This substance on evaporation remains as a brownish red deliquescent mass; on drying at 300° C. it is converted into platinous chloride, PtCl_{2}, and becomes insoluble, and at a higher temperature it is converted into platinum. All platinum compounds yield the metal in this way. Platinic chloride combines with other chlorides to form double salts, of which the ammonic and potassic platino-chlorides are the most important. Platinum alone is not soluble in nitric acid; but when alloyed with other metals which dissolve in this acid it too is dissolved; so that in gold parting, for example, if platinum was present, some, or perhaps the whole of it would go into solution with the silver. Such alloys, however, when treated with hot sulphuric acid leave the platinum in the residue with the gold.