It was dried for several days at 65°C., but for almost a month after its preparation was not used. Hence, what the ratio of the manganese to the oxygen at the time when the permanganate was treated with it, is quite uncertain. It however appears probable from the subsequent experiments on the instability of manganese dioxide at 65°C. that the ratio of manganese to available oxygen must have been at about 1.05: 1.00 corresponding to a formula MnO·20MnO₂. Two solutions of permanganate were made, filtered through glass wool and standardized. Solution A. A cubic centimetre of this was found to be equivalent to 5.278 m.g. iron in the ferrous condition. Solution B. A cubic centimetre of this was found to be equivalent to 14.379 m.g. iron in the ferrous condition. Three 250 c.c. portions of each of the solutions A and B were placed in clean glass-stoppered bottles. These bottles were labelled “a”, “a′” and “a″” “b”, “b′” and “b″”. “a” contained 250 c.c. potassium permanganate each cubic centimetre of which was equivalent to 5.278 m.g. Fe″. It was closed and put away in the dark and allowed to stand from June fourteenth until the fourteenth of the following October. Its strength was then determined and found to have declined 0.72 percent. “a′” contained the same quantity of the same permanganate solution as “a” and it was closed on the same date and placed in mildly diffused light until October fourteenth. An analysis on the latter date showed that its strength had also declined 0.72 percent. “a″” contained the same quantity of the same permanganate solution as “a” and “a′” and to this was added 0.5 gram of the manganese oxide previously referred to. The bottle was then closed and placed in diffused light beside “a′”. An analysis, October fourteenth showed that its strength had declined 34.62 percent. “b” contained potassium permanganate each cubic centimetre of which was equivalent to 14.379 m.g. Fe″. It was closed and put away in the dark beside “a”. An analysis October fourteenth showed that its strength had declined 1.71 percent. “b′” contained the same quantity of the same permanganate as “b”. It was closed and placed in mildly diffused light beside “a′”. An analysis October fourteenth showed a decline in strength of 2.41 percent. “b″” contained the same quantity of the same permanganate as “b” and “b′”. To it was added 0.5 grams of the manganese oxide. The bottle was closed and placed in diffused light beside “a′”, “a″”, and “b′”. An analysis, October fourteenth showed a decline in strength of 78.86 percent.