We generally mean by this expression that degree of Fahrenheit's thermometer which indicates the temperature of frozen water—viz. 32 above zero. If we mean any other liquid we add the name, as the freezing-point of milk, sulphuric ether, quicksilver, and so on. In Centigrade and Réaumur's instruments zero marks the freezing-point.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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