The experiment she imagined — a prism, a spectrum, and a thermometer in the dark beyond the red.
At Cirey, in the candlelit château she shared with Voltaire, Émilie du Châtelet did mathematics and physics at the highest level of her age — a woman barred from the academies, who taught herself anyway.
In 1737 she entered, in secret, the Académie des Sciences prize competition on the nature of fire. Her essay — the Dissertation sur la nature et la propagation du feu — reasoned toward something nobody could yet detect: that heat and light were entangled, and that there might be light that carried warmth but no colour the eye could catch. An invisible light, beyond the red.
She could see it in her mind. She had no way to catch it on a thermometer.
The story goes that she wanted to test it — to set thermometers along a spectrum and look for heat where there was no colour — but that every thermometer in the house was already in use. Voltaire, working on his own fire essay for the very same prize, had them all. So the experiment stayed in her head.
It would take another sixty-three years. In 1800, William Herschel passed sunlight through a prism, laid thermometers across the colours, and found the mercury climbing highest of all in the dark just past the red — the invisible light we now call infrared. The thing she had reasoned into existence, finally caught on glass.
By then she was long gone — and her last years had been spent not on invisible light but on a different, desperate labour. She was translating Newton's Principia into French, with a commentary that did not merely render the Latin but extended it — an early formulation of the conservation of energy among its pages.
Pregnant at forty-two, in an age when that was perilous, she wrote to a friend that she did not expect to survive. She worked against the clock to finish the manuscript — and she did finish it, just before she died, days after giving birth, in September 1749.
« Le travail est le seul moyen de ne pas trouver la vie ennuyeuse. »
"Work is the only way of not finding life tedious." — attributed to Émilie du Châtelet
Her Principia appeared in 1756, seven years after her death. It remains the standard French translation to this day — read by students in the country where she was never allowed into the room. She caught one invisible thing in her mind, and one monumental thing on paper, and time has proved her right about both.