SECT. IV.--_Observations upon the Combinations of Oxygen with the simple
Substances._
Oxygen forms almost a third of the mass of our atmosphere, and is
consequently one of the most plentiful substances in nature. All the
animals and vegetables live and grow in this immense magazine of oxygen
gas, and from it we procure the greatest part of what we employ in
experiments. So great is the reciprocal affinity between this element
and other substances, that we cannot procure it disengaged from all
combination. In the atmosphere it is united with caloric, in the state
of oxygen gas, and this again is mixed with about two thirds of its
weight of azotic gas.
Several conditions are requisite to enable a body to become oxygenated,
or to permit oxygen to enter into combination with it. In the first
place, it is necessary that the particles of the body to be oxygenated
shall have less reciprocal attraction with each other than they have for
the oxygen, which otherwise cannot possibly combine with them. Nature,
in this case, may be assisted by art, as we have it in our power to
diminish the attraction of the particles of bodies almost at will by
heating them, or, in other words, by introducing caloric into the
interstices between their particles; and, as the attraction of these
particles for each other is diminished in the inverse ratio of their
distance, it is evident that there must be a certain point of distance
of particles when the affinity they possess with each other becomes less
than that they have for oxygen, and at which oxygenation must
necessarily take place if oxygen be present.
We can readily conceive that the degree of heat at which this phenomenon
begins must be different in different bodies. Hence, on purpose to
oxygenate most bodies, especially the greater part of the simple
substances, it is only necessary to expose them to the influence of the
air of the atmosphere in a convenient degree of temperature. With
respect to lead, mercury, and tin, this needs be but little higher than
the medium temperature of the earth; but it requires a more considerable
degree of heat to oxygenate iron, copper, &c. by the dry way, or when
this operation is not assisted by moisture. Sometimes oxygenation takes
place with great rapidity, and is accompanied by great sensible heat,
light, and flame; such is the combustion of phosphorus in atmospheric
air, and of iron in oxygen gas. That of sulphur is less rapid; and the
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