John Tyndall: The Blue Sky and the Radiant Heat of Gases
(Part 2)
Mountains and Glaciers
“The human mind is not satisfied with observing and studying any natural occurrence alone, but takes pleasure in connecting every natural fact with what has gone before it, and with what will come after it.”_John Tyndall, The Forms of Water in Clouds and Rivers, Ice and Glaciers, (Tyndall, 2011)
During a break from his studies in Marburg, Tyndall travelled for the first time to the Alps. It was the beginning of a love affair and a lifelong association with the mountains, both as a subject of scientific study and as a mountaineer. At that time, mid 1840s-early1850s, the technique of Alpine climbing had scarcely begun to be developed. The use of rope, for example, had not yet understood. It was later, in the 1860s, after a few fatal accidents, that the rope became an indispensable accompaniment of Alpine climbing. The ice-axe also had not assumed its modern form. Maps didn’t exist either, or they were misleading, as no proper survey had not been made, particularly for the French and Italian parts of the mountains. This unexplored world of mountains fascinated many people, and quite a few of them were climbers with a scientific mind. In the next ten years, naturalists, writers and artists began to ascend the peaks of the mountains, opening up the world of Alpine exploration and tourism. It was the Golden Age of Alpinism, a name given to the decade between Alfred Wills' ascent of the Wetterhorn in 1854 and Edward Whymper's ascent of the Matterhorn in 1865, during which many major peaks in the Alps saw their first ascents.
Tyndall was passionate about the Alps and studied them at length. He used instruments to measure the winter temperatures at the summits of the mountains, and he studied the long-term growth and contraction of the glaciers and the formation of crevasses. In 1857, Tyndall and Thomas H. Huxley presented their summer explorations on a paper, titled, ‘On the Structure and Motion of Glaciers.’ They argued that although the glaciers did seem to behave in many respects as if they were viscous – a theory developed by the Professor of physics James Forbes (1809-1868), it was not viscosity, but what Tyndall called ‘fracture and regelation’, the breaking and re-freezing of the ice. To prove his theory, Tyndall carried out laboratory experiments with ice under pressure to demonstrate the effect. 1
Why the Sky is Blue
Tyndall’s long fascination with colour also begun in the Alps. He became interested in the colour blue inside shafts driven into snow, and the colours of rivers and lakes. He couldn’t find any explanation and the puzzle would preoccupied him for years. He began to experiment with light, shining beams through various gases and liquids. He knew that the white light was made up of a whole rainbow of coloured light. He used a simple glass tube to simulate the sky, with a white light at one end to represent the sun. He discovered that when he gradually filled the tube with smoke, the beam of light appeared to be blue from the side but red from the far end.
It was then that he realised that the colour of the sky is a result of light from the sun scattering around particles in the upper atmosphere. When light from the sun enters the Earth’s atmosphere, it hits all sorts of molecules (mostly nitrogen and oxygen molecules) on its way to Earth and bounces off them like a pinball. So on a clear day, if you look at any part of the sky, the light you see has been bouncing around the atmosphere before coming into your eye. If all light was scattered equally, the sky would look white. But it doesn’t. The reason is that blue has a much shorter wavelength than red or yellow and it is more likely to be scattered and get bounced that the longer ones, so to our eyes the sky looks blue. The phenomenon is called “The Tyndall Effect.”
The “Goldilocks” planet
Part of what makes Earth habitable is its natural greenhouse effect, which keeps the planet at a friendly 15°C on average. It’s all about energy balance. The Sun radiates energy towards Earth. Part of this energy that reaches the top of the Earth’s atmosphere is reflected back to space. The rest is absorbed by the Earth’s land and the oceans and warms the planet. The Earth, in the form of heat radiation, sends the same amount of energy back to space. However, some of this radiated heat is trapped by greenhouse gases contained in the atmosphere. Water vapour and carbon dioxide are the two gases that trap the most outgoing heat.
Image: Tyndall’s radiant heat apparatus. (Courtesy of the Royal Institution of Great Britain.
In the late 1850s, Tyndall decided to measure the heat absorption of the various gases, such as water vapor, ozone, nitrogen and carbonic acid (now known as carbon dioxide). His starting point was Fourier’s theory (also Claude Pouillet, the French physicist who first attempted to calculate the sun’s energy output), that the atmosphere could allow heat from the sun to pass through more easily than heat emanating back from Earth. In order to measure accurately the ability of the gases to absorb radian heat, he built a sensitive apparatus, a ratio spectrophotometer and set about examining the transmission of both radiant heat and light through various gases and vapours.2
He made various discoveries. He found that the diatomic molecules (molecules consisting of two atoms of the same element), nitrogen (N2) and oxygen (O2), are transparent to visible light and infrared radiation, but the triatomic molecules (composed of three atoms, of either the same or different chemical elements), such as water vapour (H2O) and carbon dioxide (CO2), are strong absorbers of infrared radiation.
“This aqueous vapour is a blanket more necessary to the vegetable life of England that clothing is to man. Remove for a single summer night the aqueous vapour from the air which overspreads this country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature. The warmth of our fields and gardens would pour itself unrequired into space, and the sun would rise upon an island held fast in the iron grip of frost. The aqueous vapour constitutes a local dam, by which the temperature at the earth’s surface is deepened; the dam, however, finally overflows, and we give to space all that we receive from the sun.” (Tyndall, 1861)
Tyndall argued the importance of atmospheric water vapour in maintaining the surface temperature of the Earth, particularly at night. At night the Earth’s surface cools by radiating heat out to space and greenhouse gases trap some of this terrestrial radiation. It is because of that that the night-time cooling is slow, argued Tyndall and he predicted that greenhouse warming should cause nights to warm faster than days.
It took over 130 years before his prediction was confirmed. In a paper in 2006, scientists concluded “between 1951 and 2003, over 70% of the land area sampled showed a significant increase in the annual occurrence of warm nights while the occurrence of cold nights showed a similar proportion of significant decrease.” (L
On the evening of Friday 10 June, 1859, John Tyndall delivered a lecture at the Royal Institution. It was the first public exposition of his experimental findings on radiant heat and its passage through the atmosphere (Hulme, 2009). Explaining his results to the audience, he said,
“The bearing of this experiment upon the action of planetary atmospheres is obvious ... the atmosphere admits of the entrance of the solar heat, but checks its exit; and the result is a tendency to accumulate heat at the surface of the planet.” (Tyndall, On the transmission of heat of different qualities through gases of different kinds Proceedings of the Royal Institution, 1859)
John Tyndall’s experimental work in 1859, and again in 1860/1861, became central to the subsequent understanding of the heat budget of the atmosphere. He has often been credited with the discovery of the absorption of heat by carbon dioxide and water vapour, as well as our current understanding of the greenhouse effect and global warming. (Tyndall, On Radiation through the Earth’s Atmosphere, 1872) Yet, it was an American woman, Eunice Foote, an amateur scientist and women's rights campaigner that first made the discovery, in 1856, three years before Tyndall. Her results were announced at the 1856 meeting of the American Association for the Advancement of Science and were published in the American Journal of Science and Arts the same year as “Circumstances Affecting the Heat of the Sun’s Rays.”
James Forbes was professor of physics at Edinburgh University. In 1840, he met Agassiz at the Glasgow meeting of the British Association for the Advancement of Science and become Intrigued by the new radical glacial theory, that vast sheets of ice had once covered much of the northern hemisphere. He went to Switzerland to view and study the Alpine glaciers and it was the first who developed the theory that the glaciers move by a kind of viscous flow.
Heat from a cube containing boiling water was passed through the tube that would contain his sample gas, and detected on one face of a thermoelectric pile. He used a compensating cube, which radiated against another face of the thermoelectric pile to balance the heat from the cube radiating through his sample tube. This enabled very small amounts of absorption, previously undetectable, to be measured by difference.
Sources and References:
Eve, A., Creasey, C. H., & Schuster, L. (1945). Life and Work of John Tyndall . London: MacMillan & Co.
Hulme, M. (2009). On the origin of ‘the greenhouse effect’: John Tyndall’s 1859 interrogation of nature. Weather, 64(5), 121-123. doi:https://doi.org/10.1002/wea.386
Jackson, R. (2018). The ascent of John Tyndall. Oxford: Oxford University Press.
LV Alexander, Z. X., Peterson, T., Caesar, J., Gleason, B., klein Tank, A., Haylock, M., . . . Griffiths, G. (2006). Global observed changes in daily climate extremes of temperature and precipitation. J. Geophys. ReSEARCJ.
Tyndal , J. (1861). On the absorption and radiation of heat by gases and vapours, and on the physicalconnexion of radiation, absoption and conduction. London: Philosophical Magazine.
Tyndall, J. (1859). On the transmission of heat of different qualities through gases of different kinds Proceedings of the Royal Institution. Proceedings of the Royal Institution, 155-158.
Tyndall, J. (1872). On Radiation through the Earth’s Atmosphere. In J. Tyndall, Contributions to molecular physics in the domain of radiant heat. A series of memoirs published in the 'Philosophical transactions' and 'Philosophical magazine, ' with additions. (pp. 421-424). Longmans, Green, and Co. Retrieved from https://archive.org/details/contributionsto01tyndgoog/page/n444/mode/1up?view=theater