The following experiments show the effect of variation in the conditions of the assay. Use a solution of copper sulphate containing 39.38 grams of copper sulphate crystals (CuSO_{4}.5H_{2}O) in the litre. 100 c.c. equal 1.00 gram of copper. ~Effect of Varying Temperature.~--The assay after the addition of the potassic iodide must be kept cold, else iodine may be volatilised. ~Effect of Varying Potassium Iodide.~--In various descriptions of the process the amount of iodide required is variously stated at from "a few crystals" to as much as 10 grams. The proportion required by theory for 1 gram of copper is a little over 5 grams: an excess, however, is required to keep the liberated iodine in solution. On economic grounds this excess should not be extravagant; if the student uses 10 parts of the iodide for each part of copper in the assay he will have sufficient. In the experiments there were used 20 c.c. of the copper sulphate, with varying amounts of potassic iodide, and the following results were got:-- Potassic iodide added 1.5 gram 3 grams 5 grams "Hypo" required 20.0 c.c. 20.0 c.c. 20.0 c.c. In these the iodide was added direct to the solution containing the copper, which was afterwards diluted to 100 c.c. and titrated. In another series the iodide was added after the dilution to 100 c.c., and the results were:-- Potassic iodide added 1.5 gram 3 grams 5 grams 10 grams "Hypo" required 20.0 c.c. 20.1 c.c. 20.0 c.c. 20.0 c.c. ~Effect of Varying Bulk.~--In these experiments, 20 c.c. of copper sulphate were taken, 3 grams of potassic iodide added, and also water to the required bulk. Bulk 20.0 c.c. 100.0 c.c. 200.0 c.c. 500.0 c.c. "Hypo" required 20.0 " 20.0 " 20.0 " 19.9 " In the last of these experiments the colour was discharged at 18 c.c., but gradually returned until 19.9 c.c. had been run in. It will be seen that considerable variation in bulk does not interfere. ~Effect of Acetic Acid.~--These experiments were like the last series mentioned, but the bulk was 100 c.c., and varying amounts of acetic acid were added. Acetic acid added 0 c.c. 1.0 c.c. 5.0 c.c. 10.0 c.c. 20.0 c.c. "Hypo" required 20.0 " 20.1 " 20.1 " 20.0 " 20.2 " Acetic acid, then, does not interfere to any serious extent. ~Effect of Varying Sodium Acetate.~--These experiments were like those last mentioned, but without acetic acid, and with varying amounts of sodium acetate. Sodium acetate added 0 gram 1 gram 2 grams 5 grams 10 grams "Hypo" required 20.0 c.c. 20.0 c.c. 20.2 c.c. 19.3 c.c. 18.2 c.c. In the 5 grams experiment, when the finishing point had been apparently reached the colour slowly returned; but as the results generally on titrating were not satisfactory a repetition of the experiment was made with the addition of 5 c.c. of acetic acid, which gave an equally bad result.