This ratio would correspond very nearly to a compound having the composition MnO·6MnO₂ but it is by no means certain that the reducing action of nitric acid was finished. The action of this oxide on potassium permanganate was determined under the same conditions as in the case of the brown oxide as described on [page 14]. The following illustration of the proportions used is taken from the first experiment.

Experiment No. I

Flask No. I Manganese oxide, black, = 150 m.g.
N/10 nitric acid = 14.93 c.c.
[= Mn in KMnO₄ in No. II or No. III]
Water = 27.46 c.c.
[Total solution = 42.39 c.c.]

Flask No. II Potassium permanganate = 20 c.c. = 117.71 m.g.
N/10 nitric acid = 22.39 c.c.
[= K and Mn in KMnO₄
[Total solution = 42.39 c.c.]
Flask No. III Potassium permanganate = 20 c.c. = 117.71 m.g.
N/10 nitric acid = 22.39 c.c.
[= K and Mn in KMnO₄]
Manganese oxide, black = 150 m.g.
[Total solution = 42.39 c.c.]

The first two results are from experiments in which the action was stopped at the first loss of the color of potassium permanganate. The last two are the results from experiments in which the action was stopped after the oxide had settled leaving a clear colorless supernatant liquid. The amount of oxygen given off in any one case is expressed in the table as the number of atoms of oxygen derived from each molecule of potassium permanganate, as follows:

A = N/10 nitric acid. No. of molecules.
Flask I Flask II Flask III
Black
oxide
150 m.g.		  A  Potassium
permanganate
20 c.c. =
117.71 m.g.		  A  Potassium
permanganate
20 c.c. =
117.71 m.g.		  A 
1 2 hours.
0.00 2 0.18 3 1.337 3
2 2¼ hours
0.02 ” 0.24 ” 1.335 ”
3 2½ hours
0.02 ” 0.44 ” 1.402 ”
4 4½ hours
0.00 ” 0.55 ” 1.362 ”

The volume of the oxygen evolved is considerably smaller than that obtained when the brown oxide was used. It is nevertheless larger than could have been evolved if all of the manganese had been left in the dioxide condition at the close of the experiment. This fact indicates that there was a slight reduction of the manganese dioxide after the disappearance of the color of the permanganate. That such a reduction of manganese dioxide does take place will be made apparent by a comparison of the results of the first two experiments in which the oxygen was measured soon after the color of permanganate had disappeared, with the results of the last two experiments in which the oxygen was determined after a longer interval. It will be noticed in this as in the series of experiments with the brown oxide that the rate of the evolution of oxygen liberated in flask No. II increases with the time of heating.

The Amount of Nitric Acid neutralized.

A few experiments were made to determine how much nitric acid is neutralized under the conditions which prevailed in the foregoing experiments.

I. The flask contained brown oxide 150 m.g. N/10 nitric acid 6.98 c.c. and was heated for three hours in the water-bath.