The use of flour as a reducing agent has many advantages, and it is well to remember that _1 gram of flour reduces about 11 grams of lead_; and that charcoal has twice, and tartar one-half, this reducing effect. _On the Reducing Effect of Charcoal, &c., on Red Lead._--It is often easier to obtain red lead of good quality than it is litharge, and by a large number of assayers red lead is the form of oxide of lead always used. Red lead, however, contains an excess of oxygen which will use up some of the reducing agent before lead separates out. On making a series of experiments (similar to the last, but using 80 grams of red lead instead of the litharge) the results were, with the same quantities of the reducing agents:-- With charcoal, 18 grams of lead. " flour, 18 " " " tartar, 22 " " Comparing these with the results with litharge, in the previous table it will be seen that the same quantity of reducing agent has in each case brought down 16 grams less of lead, so that a larger amount of the reducing agent must be added to get a button of the same weight as that obtained with litharge. To get a button of a desired weight, say 22 grams, we must add reducing agent sufficient to throw down 22 + 16 or 38 grams of lead, which would require 3.4 grams of flour. If this amount of flour is fused with 80 grams of red lead, a button of lead weighing 22 grams will be formed, the other 16 grams being kept up by the oxygen of the red lead. If the quantity of red lead differs from 80 grams, this rule must be modified. With 40 grams of red lead, for example, we should add an excess of reducing agent sufficient to throw down 8 grams of lead instead of 16. Similarly, with 160 grams of red lead, we should add enough to throw down 32 grams. The following rule will enable one to calculate the weight of flour required to produce a button of lead of any desired weight from any given quantity of red lead. Each 5 grams of red lead present diminishes the weight of the lead by 1 gram. If then we _divide the weight of red lead in a charge by 5, and add this to the weight of lead required, the sum divided by 11 will give the weight of flour which must be added_. Using 80 grams of red lead and wanting a button of 20 grams, we should add 3.3 grams of flour. 80/5 = 16; 16+20 = 36; 36/11 = 3.3 nearly. The following are some results obtained which will illustrate the rule:-- Red Lead used. Flour used. Lead got. 40 grams 3 grams 25.0 grams 100 " 3 " 13.5 " 80 " 4 " 30.0 " 80 " 5 " 40.0 " _On the Reducing Effect of Metallic Sulphides, and the Counteracting Effect of Nitre._--The sulphides found in ores will reduce a button of lead from oxide of lead just as flour does; and, as charcoal, flour and tartar differ in their reducing power, so equal weights of the different mineral sulphides throw down different weights of lead.