Solutions of ferrous sulphate and of ferrous chloride were made containing 0.5 gram of iron in each 100 c.c., thus corresponding to the standard solutions of permanganate and bichromate of potassium. These last were prepared in the way already described. The solution of ferrous sulphate was made by dissolving 5.01 grams of iron wire in 100 c.c. of dilute sulphuric acid and diluting to 1 litre. A similar solution may be made by dissolving 24.82 grams of pure ferrous sulphate crystals in water, adding 100 c.c. of dilute sulphuric acid, and diluting to 1 litre. ~Rate of Oxidation by Exposure to Air.~--This is an important consideration, and if the rate were at all rapid would have a serious influence on the manner of working, since exclusion of air in the various operations would be troublesome. 20 c.c. of the solution of ferrous sulphate were taken in each experiment, acidified with 10 c.c. of dilute sulphuric acid, and diluted to 100 c.c. The solution was exposed, cold, in an open beaker for varying lengths of time, and titrated with permanganate of potassium. Time exposed 1 hour 1 day 2 days 3 days c.c. required 19.2 19.1 19.0 19.0 These results show that the atmospheric oxidation in cold solutions is unimportant. With boiling solutions the results are somewhat different; a solution which at the outset required 20 c.c. of permanganate of potassium, after boiling for an hour in an open beaker (without any precautions to prevent oxidation), water being added from time to time to replace that lost by evaporation, required 19.2 c.c. If the solution be evaporated to dryness the oxidising power of concentrated sulphuric acid comes into play, so that very little ferrous iron will be left. A solution evaporated in this way required only 2.2 c.c. of permanganate of potassium. ~Effect of Varying Temperature.~--In these experiments the bulk was in each case 100 c.c., and 10 c.c. of dilute sulphuric acid were present. The permanganate required by 1 c.c. of ferrous sulphate was, at 15° 1.0 c.c., and at 70° 1.1 c.c. 10 " " " 9.7 " 9.8 " 100 " " " 97.7 " 96.8 " The lower result with the 100 c.c. may be due to oxidation from exposure. ~Effect of Varying Bulk.~--The following experiments show that considerable variations in bulk have no practical effect. In each case 20 c.c. of ferrous sulphate solution and 10 c.c. of dilute acid were used. Bulk of assay 30 c.c. 100 c.c. 500 c.c. 1000 c.c. Permanganate required 20.0 " 20.0 " 20.2 " 20.5 "