256 Proceedings of the Royal Society 
resulting oxide was weighed and analysed by determining the 
quantity of protoxide it yielded when heated in a current of 
hydrogen. Most of the experiments were carried on under ordi- 
nary atmospheric pressure ; hut in a few cases, the tension of the 
gas was made less than one atmosphere by connecting the exit end 
of the porcelain tube with a large bell -jar, within which the pres- 
sure was diminished, and kept constant by means of an air-pump. 
Eighteen experiments gave the following results. 
In all cases, either black Mng O3, or brown Mng O4 was obtained ; 
no intermediate oxides were formed. 
In all the cases in which Mng O4 was obtained, the partial tension 
of the oxygen lay between 0 and 0*21 atmospheres (the latter 
tension being that of the oxygen of the atmosphere). In all the 
experiments in which Mn2 O3 was obtained, the partial tension of 
the oxygen lay between 0*25 and one atmosphere. 
Oxygen tensions intermediate between 0*21 and 0*25 did not 
occur in the series ; but in spite of this gap, the results obtained 
render it highly probable that the function expressing the relation 
between the composition of an oxide of manganese which is formed 
at a red heat in an atmosphere of oxygen, and the tension of that 
oxygen is discontinuous, so that Mn2 03 is formed whenever the 
tension exceeds^ and Mii3 O4 whenever it is helow a certain definite 
limit. 
To determine whether such a point of discontinuity really exists, 
and if so, its position, a second series of experiments was per- 
formed, which were made as uniform, and therefore comparable, 
as possible. In all cases, the Mn O2 was heated in mixtures of 
nitrogen and oxygen of exactly known composition, kept at, or 
very near to, the tension of the surrounding atmosphere. The per- 
centages of oxygen in those mixtures were so chosen as gradually 
to inclose the value sought for within narrower and narrower limits 
In each experiment, the height of the barometer was determined, 
in order to know exactly the partial tension of the oxygen in the 
mixture. 
Eleven experiments were made in all. In eight, the oxides 
obtained were either pure Mn2 O3, or Mu3 O4. In three, the oxides 
were obtained together, not mixed, but as it were side by side, 
and occupying different parts of the boat. In those cases, in which 
