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NCERT tells you that carbon dioxide lets sunlight through but absorbs heat radiated by the Earth, and that this drives the greenhouse effect. The idea that carbon dioxide affects Earth's temperature is older than most students expect. In 1856 Eunice Foote, an American scientist, tested gases in sunlight with very simple equipment and drew that conclusion in one sentence. This post covers what she did, her numbers, a common temperature-conversion trap, and what her experiment could and could not show.
Who was Eunice Foote?
A detailed study by Ortiz and Jackson in Notes and Records of the Royal Society (2020) gives her dates as 1819 to 1888 and says she published two known papers. They describe her 1856 paper as the more significant one, because in it "she related changes in the types and amounts of atmospheric gases, including carbon dioxide, to warming and changes in climate".
What did she actually do?
Her paper, Circumstances affecting the Heat of the Sun's Rays, appeared in the American Journal of Science and Arts in November 1856, marked "Read before the American Association, August 23d, 1856." It is short, and you can read it in a few minutes.
She used an air-pump and two glass cylinders of the same size, "about four inches in diameter and thirty in length", each holding two thermometers. She filled or treated the cylinders differently, let them reach the same temperature, then set them side by side in the sun.
She reported three results:
Denser air heated more. With air pumped out of one cylinder and squeezed into the other, the condensed tube rose to 110° while the other reached only 88°. She linked this to the weak heating of the Sun on high mountains.
Moist air heated more than dry air. One cylinder was saturated with water vapour, the other dried with calcium chloride.
Carbon dioxide heated most. "The highest effect of the sun's rays I have found to be in carbonic acid gas," she wrote. That cylinder became "much heated" and "was many times as long in cooling".
Comparing gases, she found 104° in hydrogen, 106° in common air, 108° in oxygen and 125° in carbonic acid gas, the old name for carbon dioxide.
Then came her key sentence: "An atmosphere of that gas would give to our earth a high temperature". She added that if, as some supposed, the air once held more of it, a higher temperature "must have necessarily resulted".
Worked example: converting her temperatures
Foote's readings are in degrees Fahrenheit. Convert with C = (F − 32) × 5/9.
Common air, 106 °F: (106 − 32) × 5/9 = 74 × 5/9 ≈ 41.1 °C
Carbon dioxide, 125 °F: (125 − 32) × 5/9 = 93 × 5/9 ≈ 51.7 °C
So the difference is about 51.7 − 41.1 = 10.6 °C.
Now the trap. The gap in Fahrenheit is 125 − 106 = 19 F degrees. To convert a difference, you only multiply by 5/9: 19 × 5/9 ≈ 10.6 C degrees. If you subtract 32 from a difference, you get (19 − 32) × 5/9 ≈ −7.2, which is nonsense.
Try the pressure test the same way: 110 °F and 88 °F differ by 22 F degrees, which is 22 × 5/9 ≈ 12.2 C degrees.
Our post on Anders Celsius and his scale explains where the Celsius scale's fixed points came from.
What could her experiment not show?
Foote's cylinders sat in sunlight, so she measured how warm the whole set-up became: gas, glass and thermometers together. She did not separate sunlight from the heat that warm objects give off, and she noted that she had no way to measure how much she had compressed or thinned the air.
The modern explanation, as the NCERT Class 11 chapter on composition and structure of the atmosphere puts it, is that carbon dioxide is "transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation". Measuring how gases absorb that kind of heat needed different apparatus. John Tyndall's 1861 Bakerian Lecture to the Royal Society, On the absorption and radiation of heat by gases and vapours, was a major step in that direction. For the modern measurements, see our post on Charles Keeling and the Keeling curve.
What mistakes do students make with this topic?
Converting temperature differences with −32. Only absolute readings need the offset.
Overstating the result. Foote showed that carbon dioxide warmed more in her cylinders in sunlight; the full greenhouse mechanism came from later work.
Ignoring controls. Her two cylinders were the same size and started at the same temperature. Without that, the comparison would mean little.
If heat transfer by radiation is confusing, the Fermi Concept Library has every concept in physics, chemistry, biology, mathematics and computing, so you can revise radiation and absorption on their own.
What can students learn from Foote's way of working?
Foote used cheap equipment, changed one variable at a time, wrote down her readings and stated a clear conclusion. She was also honest about what she could not measure. Small, well-controlled experiments can carry big ideas.
This week, design a two-container experiment at home to compare how fast two liquids cool, and keep everything the same except the one thing you are testing.
