This
followed from his theorem that the total mechanical energy of a
flowing liquid, comprising the energy associated with fluid
pressure, the gravitational potential energy of elevation, and
kinetic energy of fluid motion remains constant; that is it is
conserved. He demonstrated that the impact of molecules on a
surface would explain pressure, and that assuming the constant
random motion of molecules, pressure and motion will increase with
temperature. He explained the behavior of gases with changing
pressure and temperature. In 1754, Scotsman physician Joseph Black
identified carbon dioxide, the first gas recognized as distinct
from everyday breathing air. He did this by using a balance to
weigh alkalis before and after exposure to heat. They lost weight
by losing carbon dioxide. He then ascertained the effects of
carbon dioxide on animals and its production by respiration,
fermentation, and burning of charcoal. At this time, all flammable
materials were thought to contain "phlogiston", which was given
off as they burned and was associated with the transfer of heat.
Plants were thought to remove phlogiston from the air and
therefore burned when they were dry. In 1774, Joseph Priestly, a
nonconformist minister, schoolmaster, and tutor, discovered oxygen
by heating red oxide of mercury. He got interested in the study of
gases by watching the process of fermentation in a brewery next to
his house.
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