On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connection of Radiation, Absorption, and Conduction
Tyndall, John, 1861. On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connection of Radiation, Absorption, and Conduction. 'Philosophical Magazine ser. 4, vol. 22, 169–94, 273–85.
Essay about this article
The original experiments on the radiative properties of various gases conducted by John Tyndall (1820-1893) at the Royal Institution of Great Britain successfully demonstrated that small amounts of “perfectly colorless and invisible gases and vapours” were able to absorb and emit amounts of radiant heat sufficient to control the heat budget of the planet.
Tyndall's apparatus, the first ratio spectrophotometer, consisted of a long tube that he filled with various gases. The ends of the tube were capped with slabs of rock salt crystal, a substance known to be highly transparent to heat radiation. A standard Leslie cube emitted radiation that traversed the tube and interacted with the gas before entering one cone of a differential thermopile. Radiation from a second Leslie cube passed through a screen and entered the other cone. The common apex of the two cones, containing the differential thermopile junction, was connected in series to a galvanometer that measured small voltage differences. The intensity of the two sources of radiation entering the two cones could be compared by measuring the deflection of the galvanometer, which is proportional to the temperature difference across the thermopile. Different gases in the tube would cause varying amounts of deflection of the galvanometer needle. If the intensity of the reference source of radiation was known, the intensity of the other source (and thus the absorptive power of the gas in the tube) could be calculated. Using his apparatus, Tyndall was able to identify atmospheric trace constituents as efficient absorbers of long wave radiation and as important factors in climatic control. Specifically, he established beyond a doubt that water vapor, among the constituents of the atmosphere, was the strongest absorber of radiant heat and was the most important gas controlling the Earth's surface temperature. Its radiative properties were also important in explaining meteorological phenomena such as the formation of dew and clouds. Concerning climate, Tyndall wrote that changes in the amount of any of the radiatively active constituents of the atmosphere—water vapor, carbon dioxide, ozone, or hydrocarbons—could have produced “all the mutations of climate which the researches of geologists reveal . . . they constitute true causes, the extent alone of the operation remaining doubtful.” Tyndall's carefully executed laboratory experiments clearly demonstrated that trace atmospheric constituents were active absorbers of heat radiation. These results kept alive what was called the "hot-house theory," and they suggested to Arrhenius, Callendar, and others that the Earth's heat budget could be controlled by changes in the carbon dioxide content of the atmosphere.
See also: Tyndall, John, 1863. On Radiation through the Earth's Atmosphere. Phil. Mag. ser. 4, vol. 25, 200-206.
a. Who was John Tyndall, what were his scientific interests, and why is he considered both a great scientist and a great popularizer of science? How are the two roles interrelated?
b. Referring to the illustration at the end of the article, discuss the components of Tyndall’s experimental apparatus and its functioning.
c. Tyndall’s experiments and Darwin’s Origin of Species both date to 1859. Discuss the reception and development of the two theories of greenhouse warming and biological evolution since then.
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