CHAP. VII.
_Of the Decomposition of Oxygen Gas by means of Metals, and the
Formation of Metallic Oxyds._
Oxygen has a stronger affinity with metals heated to a certain degree
than with caloric; in consequence of which, all metallic bodies,
excepting gold, silver, and platina, have the property of decomposing
oxygen gas, by attracting its base from the caloric with which it was
combined. We have already shown in what manner this decomposition takes
place, by means of mercury and iron; having observed, that, in the case
of the first, it must be considered as a kind of gradual combustion,
whilst, in the latter, the combustion is extremely rapid, and attended
with a brilliant flame. The use of the heat employed in these operations
is to separate the particles of the metal from each other, and to
diminish their attraction of cohesion or aggregation, or, what is the
same thing, their mutual attraction for each other.
The absolute weight of metallic substances is augmented in proportion to
the quantity of oxygen they absorb; they, at the same time, lose their
metallic splendour, and are reduced into an earthy pulverulent matter.
In this state metals must not be considered as entirely saturated with
oxygen, because their action upon this element is counterbalanced by the
power of affinity between it and caloric. During the calcination of
metals, the oxygen is therefore acted upon by two separate and opposite
powers, that of its attraction for caloric, and that exerted by the
metal, and only tends to unite with the latter in consequence of the
excess of the latter over the former, which is, in general, very
inconsiderable. Wherefore, when metallic substances are oxygenated in
atmospheric air, or in oxygen gas, they are not converted into acids
like sulphur, phosphorus, and charcoal, but are only changed into
intermediate substances, which, though approaching to the nature of
salts, have not acquired all the saline properties. The old chemists
have affixed the name of _calx_ not only to metals in this state, but to
every body which has been long exposed to the action of fire without
being melted. They have converted this word _calx_ into a generical
term, under which they confound calcareous earth, which, from a neutral
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