One gram of iron pyrites yields about 11 grams of lead. One gram of copper pyrites, blende, fahlerz, or mispickel, yields 7 or 8 grams of lead, whilst 1 gram of antimonite will give 6, and 1 gram of galena only a little over 3 grams. It is evident that if an ore carries much of these sulphides, the quantity of lead reduced will be very much larger than that required for an assay. To counteract this effect nitre is added; _1 gram is added for each 4 grams of lead in excess of that required_. For example: with a 20-gram charge of an ore containing 50 per cent. of pyrites, if no nitre were added, 110 grams of lead would be got; or, if there was not sufficient oxide of lead to yield this quantity of metal, the button would be sulphury. To reduce the weight of the button by 80 grammes, we should add 20 grams of nitre, if litharge were used; or if red lead were used, we should add 16 grams of nitre, _since the oxidizing effect of 20 grams of red lead is equivalent to that of 1 of nitre_, and since 80 grams of red lead are generally used in a charge. Two assays of an ore of this kind with these quantities of nitre gave 26.0 grams of lead with litharge, and 22.5 grams with red lead. It is best to use in these assays 80 grams of red lead, 20 of soda, and 5 of borax, with 20 grams of the ore. If the lead got by the preliminary fusion in a small crucible with litharge (described under "_ores containing much sulphides_") is known, the following table will indicate the quantity of nitre, or flour, to be added with this charge:-- -------------------------------+-------------------+------------------ Lead got in Preliminary Fusion | Flour to be added | Nitre to be added with 3 grams of Ore. | to the Assay. | to the Assay. -------------------------------+-------------------+------------------ 0.0 gram | 3.3 grams | none 3.0 grams | 1.3 gram | -- 6.0 " | none | 4.0 grams 9.0 " | -- | 9.0 " 12.0 " | -- | 14.0 " 15.0 " | -- | 19.0 " 18.0 " | -- | 24.0 " 21.0 " | -- | 29.0 " -------------------------------+-------------------+------------------