What did the Réaumur scale measure?

The Réaumur temperature scale was established in 1730 by the French naturalist René-Antoine Ferchault de Réaumur (1683–1757), with its zero set at the freezing point of water and its 80° mark at the boiling point of water at normal atmospheric pressure. Use of the Réaumur scale was once widespread, but by the late 19th century it had been supplanted by other systems.

Réaumur's thermometer contained diluted alcohol (ethanol) and was constructed on the principle of using 0° for the melting temperature of water, and graduating the tube into degrees, each of which was one-thousandth of the volume contained by the bulb and tube up to the zero mark. He suggested that the concentration of alcohol employed be such that it began boiling at 80 °Ré, that is, when it had expanded in volume by 8%. He chose alcohol instead of mercury because it expands more visibly, but this posed problems: His original thermometers were very bulky, and the low boiling point of alcohol made them unsuitable for many applications. Instrument-makers generally chose different liquids, and then used 80 °Ré to signify the boiling point of water, causing much confusion.

In, 1772 Jean-André Deluc studied several substances then used in thermometers and concluded that mercury thermometers were the best for practical use because among these substances its volume varied the most linearly with the method of mixtures.From the late 18th century mercury was used almost without exception until the late 20th century.