257 
of Edinburgh, Session 1863-64. 
M 1 I 3 O 4 was obtained alone, or at least in which it predominated, 
the partial tension of the oxygen (when measured by the height of 
the equivalent column of mercury) did not exceed 7*07 inches. 
In those cases in which Mn^ O 3 was obtained, or at least pre- 
dominated, the partial tension of the oxygen was equal to, or 
greater than, 6’9 inches. Thus the point of discontinuity lies 
between the two limits 6’9 and 7*07 inches. 
Within these two tensions, sometimes the one and sometimes 
the other oxide was obtained. This fact does not speak, however, 
against the existence of some definite point of discontinuity, as the 
composition of the resulting oxide depends on the temperature at 
which the experiment is performed, as well as the tension of the 
oxygen employed. A diminution of temperature is probably of 
the same effect as an increase in the tension of the oxygen, and 
vice versa. Thus, if the tension of the oxygen in a given mixture 
of that gas and nitrogen is just equal to that corresponding to the 
point of discontinuity, for. a definite temperature t, a temperature 
slightly above that will cause the formation of Mn 3 O 4 , and one 
slightly below will produce Mu 2 O 3 . 
In one experiment I succeeded in producing the two oxides by 
means of the same mixture of nitrogen and oxygen, only the 
tension being slightly varied. Mn 2 O 3 , heated in this mixture at 
the atmospheric pressure of 29*9 inches, remained unchanged, 
while at the tension of 29'5 it was reduced to Mug O 4 . This is 
quite within the limit of ordinary barometrical variations. 
10. Notice of Glacial Clay, with Arctic Shells, near Errol, on 
the Tay. By the Kev. Thomas Brown, F.K.S.E. ^ 
The author referred to the paper on the Elie Glacial Clay, with 
Shells, read by him on the 2d of March 1863. During the succeed- 
ing summer he learned from Dr M‘Bain, that shells had been found 
in a brickfield near Errol, but so badly preserved, that none of the 
species had been determined. During the autumn, while staying 
in the neighbourhood of Perth, he took occasion to visit the Errol 
brickfield, and found that the shells, which occur in considerable 
abundance, are precisely the same group with those at Elie. The 
Leda truncata, for example, which is the characteristic shell of the 
