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In the seventeen hundreds phlogiston was thought to be an
unobservable substance which was the counterbalance of calx when
things burned. If a substance was dephlogisticated, then it had
lost its flammable phlogiston and was now in the more essential
form, its calx. It was Lavoisier, the Father of Modern
Chemistry, who refuted the phlogiston theory of combustion.
Lavoisier in 1789 published a book, Traité élémentaire de
chemie, in which he purported his Oxygen Theory to refute
the ruling idea of the existence of a substance called
phlogiston, and which had been used for almost a century up to
that time to explain how substances burn. Two other
scientists in the time of Lavoisier had isolated oxygen
chemically, Carl Scheele in 1772 and Joseph Priestley in 1774.
Scheele had called the substance 'fire air.' Priestley
called the substance he had isolated by directing sunlight
through a lens onto mercurius calcinatus (mercuric oxide, a red
powder) so as to heat it, "dephlogisticated air."
The equation for Priestley's experiment would be as follows:
2HgO(s) + heat ---> 2Hg(l)
+ O2
Lavoisier developed an accurate scale which could measure
quantities of mass in the range of the nearest 10-4 g
(to 0.0005g). He then performed several experiments with
combustion, and found that there was a conservation of mass
ultimately, that the mass of the reactants and the mass of the
products remained unchanged by the combustion. This became
part of the key to his newly found understanding that air was
comprised of oxygen, nitrogen and carbon dioxide. In a
meeting with Priestley he was told of Priestley's discovery of
dephlogisticated air in the experiment Priestley had performed.
This had inspired Lavoisier to rethink air not as losing a given
substance in the process of combustion, the purported substance
called phlogiston, as much as being comprised of one or more
substances which had reactive abilities. Indeed, Lavoisier
coined the term 'oxygen' to describe the substance which was
most active in the combustion events he had witnessed.
Lavoisier was astute enough as an empirical scientist to
recognize that the oxidation of metals and the respiration of
living things, as well as combustion, all were examples of the
same type of reaction which he had been able to analyze through
the primary tool of accurate measurements in the weight of
reactants and products in his combustion experiments.
Along with his theory of oxygen Lavoisier established the nature
of an element for the annals of chemistry, whereby an element
was defined as a substance which could not be broken down
further. He named 33 such elements, and which were
described in his book, Traité élémentaire de chemie.
Lavoisier's great breakthrough in refuting the theory of
phlogiston came when he recognized that a substance, the liquid
mercury, could change state upon being heated in a closed
container to solid mercuric oxide, reducing the volume of air in
so doing; then it could be changed back with further heating
from that so-called "calx" of mercury, to make
the explanation of phlogiston now untenable. Phlogiston
could be released upon the first heating, but not put back when
the 'calx' was re-heated to give the original liquid mercury
plus O2, since according to the existing theory of
the nature of phlogiston such a replenishment should have been
deemed impossible.
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