~Titrations in Hydrochloric Solutions.~--These are less satisfactory than those in sulphuric solutions, since an excess of hydrochloric acid decomposes permanganate of potassium, evolving chlorine, and since the finishing point is indicated, not by the persistence of the pink colour of the permanganate, but by a brown coloration probably due to perchloride of manganese. Nevertheless, if the solution contains only from 5 to 10 per cent. of free hydrochloric acid (sp. g. 1.16) the results are the same as those obtained in a sulphuric acid solution. Equal weights (0.1 gram) of the same iron wire required exactly the same quantity of the permanganate of potassium solution (20 c.c.) whether the iron was dissolved in dilute sulphuric or dilute hydrochloric acid. The following series of experiments are on the same plan as those given above with sulphuric acid solutions. A solution of ferrous chloride was made by dissolving 5.01 grams of iron wire in 50 c.c. of dilute hydrochloric acid and diluting to 1 litre. The dilute hydrochloric acid was made by mixing equal volumes of the acid (sp. g. 1.16) and water. ~Rate of Atmospheric Oxidation.~--20 c.c. of the ferrous chloride solution were acidified with 10 c.c. of the dilute hydrochloric acid and diluted to 100 c.c. This solution was exposed cold in open beakers. Time exposed -- 1 hour 1 day 2 days 3 days Permanganate required 19.6 c.c. 19.6 c.c. 19.5 c.c. 19.4 c.c. 19.5 c.c. Similar solutions boiled required, before boiling, 20 c.c.; after boiling for one hour, replacing the water as it evaporated, 19.3 c.c.; and after evaporation to a paste and redissolving, 17.0 c.c. ~Effect of Varying Temperature.~--Solutions similar to the last were titrated and gave the following results:-- Temperature 15° 30° 50° 70° Permanganate required 19.8 c.c. 19.6 c.c. 19.5 c.c. 19.4 c.c. ~Effect of Varying Bulk.~--As before, 20 c.c. of the iron solution, and 10 c.c. of the dilute acid were diluted to the required volumes and titrated. Bulk 30 c.c. 100 c.c. 500 c.c. 1000 c.c. Permanganate required 20.4 " 20.3 " 20.8 " 21.5 " The variation due to difference in bulk here, although only equal to an excess of 0.7 milligram of iron for each 100 c.c. of dilution, are about three times as great as those observed in a sulphuric acid solution. ~Effect of Free Hydrochloric Acid.~--In these experiments 20 c.c. of the ferrous chloride solution were used with varying quantities of acid, the bulk of the assay in each case being 100 c.c. Dilute acid present 5 c.c. 10 c.c. 50 c.c. 100 c.c. Permanganate required 20.2 " 20.2 " 20.5 " 21.0 " The last had a very indistinct finishing point, the brown coloration being very evanescent. The effect of the acid is modified by the presence of alkaline and other sulphates, but not by sulphuric acid. Repeating the last experiment we got--