~Determination of Copper in Copper Pyrites.~--Weigh up 2 grams of the dried and powdered ore, and place in an evaporating dish about four inches in diameter. Cover with 20 c.c. of nitric acid and put on a hot plate. Evaporate to dryness without further handling. Allow to cool and take up with 30 c.c. of hydrochloric acid, boil, dilute, and transfer to a pint flask, filtering if necessary. Make up the bulk with the washings to about 150 c.c. Precipitate with sulphuretted hydrogen, filter, and wash back the precipitate into the flask. Add 15 c.c. of nitric acid, and boil down rapidly to 10 c.c. Dilute, add 30 c.c. of dilute ammonia, make up to 150 c.c., and cool. For the standard, weigh up 0.5 gram of copper, more or less, according to the quantity judged to be present in the assay. Dissolve in 20 c.c. of dilute nitric acid, boil off nitrous fumes, add 30 c.c. of dilute ammonia, make up to the same bulk as that of the assay, and cool. Titrate the two solutions side by side and as nearly as possible in the same manner. Since the assay solution is often turbid from the presence of small quantities of lead and of iron from incomplete washing, and since this slight precipitate is very slow in settling, the standard can hardly be compared strictly with the assay. This can be counteracted by precipitating in both solutions a mixture of ferric and aluminic hydrates, which settles readily and leaves the supernatant liquor clear. To effect this, boil the nitric acid solutions with 30 c.c. of a solution containing 15 grams each of alum and ferrous sulphate to the litre. In an actual determination 2 grams of the ore were taken and compared with 0.5 gram of copper. The assay required 57.7 c.c. of cyanide and the standard 52.5 c.c. 52.5 : 0.5 :: 57.7 : 0.5495 This on 2 grams of ore = 27.47 per cent.; the same sample by electrolysis gave 27.60 per cent. of copper. ~Determination without Previous Separation.~--Dissolve up 2 grams as before, but, instead of passing sulphuretted hydrogen, add 30 c.c. of dilute ammonia, shake well, and cool. Prepare a standard by dissolving 0.5 gram of copper in 1 c.c. of nitric acid, add 0.6 gram of iron in the form of ferric chloride and 20 c.c. of hydrochloric acid, dilute to about 150 c.c., add 30 c.c. of dilute ammonia, and cool. Titrate the two solutions side by side. In a determination on the sample last used, 58 c.c. were required for the assay and 53 c.c. for the standard, which indicates 27.3 per cent. of copper. This method of working is somewhat rough. IODIDE METHOD.