255 
of Edinburgh^ Session 1863-64. 
to agree approximately with the observed angle. If this should 
prove to he generally the case, it will not only be a solution of the 
problem which forms the subject of this note for crystals of the 
oblique system, but it will prove that these crystals are formed 
according to the same laws of symmetry as crystals of the prismatic 
system. 
9. A Contribution to the History of the Oxides of 
Manganese. By W. Hittmar, Esq. 
It is known that oxides of manganese, when ignited in air, either 
absorb or lose oxygen, until they attain the composition expressed 
by the formula Mu3 O4. Schneider found that when they are 
heated in pure oxygen, they leave the oxide Mn2 O3. 
This latter result is not what one might have expected, for we 
know of no positive action exerted on manganese oxides or on pure 
oxygen by nitrogen. Thus air should act like dilute oxygen, pro- 
ducing the same oxide. 
To explain Schneider’s result, we must suppose either that 
artificial oxygen differs from that contained in air, or, what is more 
likely, that the amount of oxygen which any given quantity of 
manganese can take up when heated in it, depends not only on the 
temperature, but also on the tension of the oxygen employed. 
If, then, it is true, that at a red heat, in presence of oxygen of 
the tension of one atmosphere, Mn2 O3 is a stable compound, while 
the same oxide, exposed to the same temperature in presence of 
oxygen of the tension of one-fifth of an atmosphere, is reduced to 
Mog O4, the question naturally arises, What oxides will be formed 
in presence of oxygen of different tensions from those two above 
mentioned ? Is there a continuous series of oxides corresponding 
to a continuous series of oxygen-tensions under which they have 
been formed ? 
These questions seemed to me to merit investigation, especially 
as their results would form an addition to our very imperfect know- 
ledge of the influence of physical conditions on chemical reactions. 
In the first series of experiments, pure peroxide of manganese, 
in a porcelain or platinum boat, was placed in a porcelain tube, 
kept at a bright red heat, while a current of nitrogen or oxj^gen, 
or of a mixture of these two gases, was passed over it. The 
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