Réaumur temperature scale

In 1730 René Antoine Ferchault de Réaumur, a distinguished French scientist, proposed a scale that depended on only one endpoint, the freezing point of water, which was taken as zero. The thermometer was filled to the zero mark with a solution of alcohol and water at the temperature of the freezing point of water. The proportion of alcohol to water in the solution was chosen such that if its volume at the freezing point of water was 1000, its volume at the solution's boiling point was 1080. This temperature, the boiling point of the solution, was 80 degrees on Réaumur's scale.

As a practical matter, Réaumur's method of calibration was unworkable, and in practice by 1770 the instrument makers were making mercury thermometers using two end points (0 as the freezing point of water and 80 degrees the boiling point of water), dividing the space between them into 80 intervals, and calling them Réaumur degrees. Thus one Réaumur degree is 1¼th of a degree centigrade, and a temperature in degrees Réaumur is 80% of the temperature in degrees centigrade.

This scale was adopted in France and most of Europe, excluding only Britain and Scandinavia, prior to the metric system.

On 12 Germinal an II (1 April 1794) the revolutionary French government instituted the centigrade scale as part of the new, decimal, metric system, thus doing away with the Réaumur scale.

For Further Reading

W. E. Knowles Middleton.
A History of the Thermometer and its Use in Meteorology.
Baltimore, MD: Johns Hopkins Press, 1966.