Learners Ideas and Misconceptions - Essentials of Chemical Education - page 5

using a balance did not support his theory: “He did not ascribe importance to the
weight increase of the formed metal lime, instead he tried to dismiss this problem
with the assumption of a 'negative weight' of the phlogiston and chemists were
mostly focused on the qualitative phenomena, not on the weight of substances. The
Phlogiston theory turned out to be an excellent model for explanation and systema-
tization of qualitative conversions of matter” [ 1 ].
The view of the supporters of the Phlogiston theory becomes clearer, when it is
seen from the point of energies rather than substances. Not only the flames and
substances can be observed during combustion, but also thermal energy. So if one
replaces the “substance Phlogiston” by heat or energy, many observations of the
combustion process can be explained better [ 3 ]. In this context people often thought
of a special “heat substance” that is leaving the burning fuel; energy was thought of
as matter, as substantial.
Lavoisier threw light on this issue with measurements of masses observed for the
synthesis and decomposition of mercury oxide. For this decomposition he took
even a glass bowl with water, fixed a special volume of air under a glass cover and
observed that the volume decreased by 20% (see Fig. 1.1 ). He did not only find
oxygen as a new element and the oxidation theory, but also the law of conservation
of mass. Prior to these results of Lavoisier oxygen had also been called “fire
substance” (Empedocles), “Phlogiston” (Stahl), and “fire air” (Scheele). “A rem-
nant of these terms was still in use as 'heat substance' for several decades, until the
reason for heat was found in the movement of smallest particles” [ 1 ].
Horror vacui and air pressure . In ancient times experiments with pipettes and
wine cradles drew the natural philosophers' attention to the nonexistence of an
absolute vacuum on earth. As soon as a substance leaves a space, another substance,
mostly air, takes its place. “In this context a statement from Canonicus came to
be known, which says that nature has a horror vacui, an aversion against the
vacuum” [ 5 ].
Even Galilee was aware of this phenomenon and knew from well builders that it
is not possible to pump up water from a depth of more than 10 m. He assumed this
measure to be the uttermost power of nature to prevent a vacuum. He invented a
Fig. 1.1 Apparatus of
Lavoisier to form mercury
oxide and to determine the
amount of used air