sufficiently renewed; but if, on the other hand, too free a circulation
were given to the external air, it would carry off the mercury in
solution in the state of vapour, so that in a few days none would remain
in the vessel.
As, of all the experiments upon the oxydation of metals, those with
mercury are the most conclusive, it were much to be wished that a simple
apparatus could be contrived by which this oxydation and its results
might be demonstrated in public courses of chemistry. This might, in my
opinion, be accomplished by methods similar to those I have already
described for the combustion of charcoal and the oils; but, from other
pursuits, I have not been able hitherto to resume this kind of
experiment.
The oxyd of mercury revives without addition, by being heated to a
slightly red heat. In this degree of temperature, oxygen has greater
affinity to caloric than to mercury, and forms oxygen gas. This is
always mixed with a small portion of azotic gas, which indicates that
the mercury absorbs a small portion of this latter gas during oxydation.
It almost always contains a little carbonic acid gas, which must
undoubtedly be attributed to the foulnesses of the oxyd; these are
charred by the heat, and convert a part of the oxygen gas into carbonic
acid.
If chemists were reduced to the necessity of procuring all the oxygen
gas employed in their experiments from mercury oxydated by heat without
addition, or, as it is called, _calcined_ or _precipitated_ per se, the
excessive dearness of that preparation would render experiments, even
upon a moderate scale, quite impracticable. But mercury may likewise be
oxydated by means of nitric acid; and in this way we procure a red oxyd,
even more pure than that produced by calcination. I have sometimes
prepared this oxyd by dissolving mercury in nitric acid, evaporating to
dryness, and calcining the salt, either in a retort, or in capsules
formed of pieces of broken matrasses and retorts, in the manner formerly
described; but I have never succeeded in making it equally beautiful
with what is sold by the druggists, and which is, I believe, brought
from Holland. In choosing this, we ought to prefer what is in solid
lumps composed of soft adhering scales, as when in powder it is
sometimes adulterated with red oxyd of lead.
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