On the Absorption and Radiation of Heat by Gases and Vapours
He measured in the lab that water vapour and CO₂ trap heat, while the air's main gases do not.
In a London basement around 1859, John Tyndall sent a beam of heat down a brass tube and discovered that the gas keeping our planet warm is not the air's main ingredient at all — but a faint trace of vapour and a whiff of carbon dioxide.
The big idea
Air is mostly nitrogen and oxygen. Tyndall let radiant heat — the invisible warmth a hot object gives off — pass through tubes of different gases, one at a time, and measured how much each one soaked up. Pure nitrogen, oxygen, hydrogen and dry air let the heat sail straight through, as if the tube were empty.
But carbon dioxide drank up the heat, and water vapour drank up even more. So the gases that trap the Earth's warmth are exactly the rare ones — the trace of carbon dioxide and the dampness in the air — not the bulk of the atmosphere. That is why the planet stays warm: those few gases act like a blanket for its heat.
How it came about
Tyndall was a mountaineer and glacier-watcher as well as a physicist, and he was fascinated by where ice ages came from. To get at it he needed to know whether the atmosphere itself could hold in heat — something most scientists doubted, assuming gases were simply transparent to it.
He built an exquisitely sensitive apparatus: a beam of heat, a long tube, and a thermopile linked to a galvanometer whose needle swung at the faintest change. The hardest part was cleanliness — stray water vapour clinging to the glassware absorbed heat all on its own and nearly fooled him. Once he dried everything properly, the verdict was unmistakable: the simple gases were transparent, the compound gases were not, and humid air was a far better heat-trap than dry. He announced it at the Royal Institution in 1859 and laid it out in full as the 1861 Bakerian Lecture.
Why it mattered
Tyndall turned the greenhouse effect from a hunch into a measured fact. Joseph Fourier had guessed decades earlier that the air keeps the Earth warmer than empty space; Tyndall showed, gas by gas, which gases do it and how strongly. He even wrote that because water vapour and carbon dioxide do the trapping, any change in their amount must change the climate — the seed of everything we now understand about a warming world.
A way to picture it
Think of a window made of a special glass that lets light in but won't let warmth back out. Sunlight streams through and heats the room; the warm room glows with invisible heat, and the glass holds that heat in. Nitrogen and oxygen are like an ordinary open window — heat passes right through. Carbon dioxide and water vapour are the special glass. There isn't much of that glass in the air, but it's the part that decides how warm the room — the whole planet — ends up.
Where it sits
Tyndall built on Joseph Fourier, who in the 1820s first asked why the Earth is warmer than it ought to be. Three years before Tyndall, the American Eunice Foote had shown sunlight heats CO₂-rich air the most and even suggested more of it would warm the world — a priority now recognised. Tyndall supplied the precise laboratory measurement; Svante Arrhenius then turned it into a number in 1896; and Charles Keeling began measuring the real rise of CO₂ in 1958. This brass tube is where that chain begins.
if, as the above experiments indicate, the chief influence be exercised by aqueous vapour, every variation of this constituent must produce a change of climate.