77
Black is shown here visiting the
engineer James Watt at his workshop
in Glasgow. Watt is demonstrating one
of his steam-powered instruments.EXPANDING HORIZONS
212°F (100°C). Then the water
begins to boil, but the temperature
does not change, even though heat
is still going into the water. Black
realized that the heat is needed to
turn the liquid into vapor—or, in
modern terms, to give the molecules
enough energy to escape from
the bonds that hold them fast in the
liquid. This heat does not change
the temperature, and seems to
disappear—so Black called it latent
heat (from the Latin for “hidden”).
More precisely, it is the latent
heat of evaporation of water. This
discovery was the beginning of the
science of thermodynamics—the
study of heat, its relation to energy,
and the conversion of heat energy
into motion to do mechanical work.
Water has an unusually high
latent heat, meaning that liquid
water will boil for a long time before
it all turns into gas. This is why
steaming is such an effective way
of cooking vegetables, why steam
has terrible scalding power, and
why it is used in heating systems.
Melting ice
Just as heat is needed to turn water
into steam, so it is needed to turn
ice into water. The latent heat of
melting ice means that ice will cool
a drink. To melt the ice requires
heat, and this heat is extracted
from the drink in which it floats,
thus cooling down the liquid.
Black explained all this to the
distillers, although he was unable
to help them save money. He also
explained it to a colleague called
James Watt, who was trying to
figure out why steam engines were
so inefficient. Subsequently, Watt
came up with the idea of the
separate condenser, which
condensed the steam without
cooling the piston and cylinder.
This made the steam engine a
far more efficient machine, and
made Watt a rich man. ■See also: Robert Boyle 46–49 ■ Joseph Priestley 82–83 ■ Antoine Lavoisier 84 ■
John Dalton 112–13 ■ James Joule 138
Joseph Black
Born in Bordeaux, France,
Joseph Black studied medicine
at the universities of Glasgow
and Edinburgh, conducting
chemical experiments in the
laboratory of his professor.
In his 1754 doctoral thesis,
Black showed that when chalk
(calcium carbonate) is heated
to become quicklime (calcium
oxide), it does not gain some
fiery principle from the fire, as
was commonly believed, but
loses weight. Black realized
that this loss must be a gas,
since no liquid or solid was
produced, and called it “fixed
air” because it was an air (gas)
that had been fixed in the
chalk. He also showed that
fixed air (which we now know
as carbon dioxide) was among
the gases that we exhale.
While professor of medicine
at Glasgow from 1756, Black
conducted his landmark
research on heat. Although
he did not publish his results,
his students circulated his
findings. After moving to
Edinburgh in 1766, he gave up
research to focus on lecturing
and—as the Industrial
Revolution gathered speed—
advising on chemical-based
innovations in Scottish
industry and agriculture.