Illustration of Réaumur scale

1730 Réaumur scale

The book of science

Tom Sharp

René Antoine Ferchault de Réaumur thermometry Illustration of Réaumur scale

Réaumur scale

If you have a thermometer with a large bulb filled with alcohol (but not 200 proof, it’s mixed with water, who knows how much), and the zero of your scale is at the point where water freezes and it takes eight percent of the alcohol in the bulb and tube up to zero to expand with the temperature to the point where the alcohol boils, then you have duplicated the scale of de Réaumur. Otherwise, René Antoine Ferchault de Réaumur is known as a mathematician who improved steelmaking, and analyzed the differences between iron and steel and developed a method of coating iron with tin, who described a method of incubating eggs, studied the relation between temperature and the growth of insects, who wrote about spiders and starfish and corals (proving that corals are animals and not plants), and published multiple volumes on the the natural history of moths and caterpillars, bees, ants, flies, earwigs, parasites, and other insects.


The Réaumur thermometers were no good for temperatures greater than the boiling point of alcohol, which would depend on the kind of alcohol. And their large bulbs made them too big to fit under a tongue. Moreover, today, outside of certain cheesemakers in Italy and Switzerland, few could tell you how warm 25 °Ré is, (although I can say it wouldn’t be too far from 25 °C).


You have not made a Réaumur scale by using a bulb of mercury and measuring eighty degrees to the boiling point of water, which apparently some have claimed to do.