Reaumur to Delisle

The Réaumur scale was proposed by René Antoine Ferchault de Réaumur in 1730. Réaumur was a French scientist primarily known for his contributions to natural history, particularly entomology. His thermometer used dilute alcohol and was calibrated between the freezing and boiling points of water, with the scale divided according to the actual expansion of the alcohol.

Réaumur calibrated his scale based on the physical expansion of his thermometric fluid — a specific dilute alcohol mixture. Between water's freezing and boiling points, the fluid expanded by exactly 80 parts in 1000 of its volume at freezing. He used this empirical measurement directly as the degree count, making 80°Ré the boiling point. The number 80 was not chosen arbitrarily but measured.

The French Revolution's embrace of the metric system in the 1790s demanded decimal-friendly units. Celsius's 0-to-100 scale fit the decimal philosophy perfectly; Réaumur's 0-to-80 did not. The French Academy of Sciences standardized Celsius for scientific work, and it spread through Napoleonic Europe. Réaumur lingered in German-speaking regions and Russia into the mid-19th century but eventually yielded everywhere. The irony is that Réaumur was French — his own country was the first to abandon his scale.

Multiply the Réaumur value by 5/4 (or 1.25). For example, 16°Ré × 1.25 = 20°C. To convert Celsius to Réaumur, multiply by 4/5 (or 0.8). The two scales share the same zero (0°C = 0°Ré); they differ only in degree size, since 100°C = 80°Ré.

Classic French confectionery texts list sugar syrup stages in Réaumur: "thread" stage at 80°Ré (100°C), "soft ball" at 94°Ré (117°C), "hard crack" at 124°Ré (155°C). Pastry chefs memorized these benchmarks and tested with a thermometer or by dripping syrup into cold water. Some vintage French and Swiss recipe books still reference Réaumur temperatures — modern reprints add Celsius equivalents in brackets. It is the last practical domain where Réaumur had a lingering foothold.

Delisle calibrated his thermometer by observing how mercury contracted as it cooled from boiling water. He measured degrees of cooling rather than degrees of warmth — the scale counted how far the temperature had dropped from the boiling point. This made 0°De the starting reference (boiling) and larger numbers represent lower temperatures, the opposite of every other common scale.

The freezing point of water is 150°De. The boiling point is 0°De. The scale spans 150 degrees between freezing and boiling — contrast with 100 degrees in Celsius or 180 degrees in Fahrenheit. To convert: °De = (100 − °C) × 1.5, so 0°C gives (100 − 0) × 1.5 = 150°De.

Joseph-Nicolas Delisle invented the scale in St. Petersburg in 1732, where he had been invited to work by Peter the Great. The scale was adopted by Russian scientists and used throughout the 18th century, particularly by Mikhail Lomonosov. It was later standardized by Joseph-Nicolas's colleague Joseph-Adam Braun, who fixed the two reference points.

Subtract the Delisle value from 100, then multiply by 2/3. Formula: °C = 100 − (°De × 2/3). For example, 150°De → 100 − (150 × 2/3) = 100 − 100 = 0°C (freezing). For 0°De → 100 − 0 = 100°C (boiling). Remember the scale is inverted: higher Delisle = colder temperature.

Delisle is the only well-known inverted temperature scale, but the concept appears elsewhere. Astronomical magnitude runs backwards (brighter stars have lower numbers). The Scoville scale for chilli heat is not inverted but is logarithmically compressed in a way that confuses people similarly. In thermometry, Delisle stands alone — every other scale (Celsius, Fahrenheit, Kelvin, Rankine, Newton, Rømer, Réaumur) increases with heat. The inversion made Delisle unintuitive, which is a key reason it lost out to Celsius.