<P> Anders Celsius, a Swedish scientist, devised the Celsius scale, which was described in his publication The origin of the Celsius temperature scale in 1742 . </P> <P> Celsius used two fixed points in his scale: the temperature of melting ice and the temperature of boiling water . This wasn't a new idea, since Isaac Newton was already working on something similar . The distinction of Celsius was to use the condition of melting and not that of freezing . The experiments for reaching a good calibration of his thermometer lasted for 2 winters . By performing the same experiment over and over again, he discovered that ice always melted at the same calibration mark on the thermometer . He found a similar fixed point in the calibration of boiling water to water vapour (when this is done to high precision, a variation will be seen with atmospheric pressure; Celsius noted this). At the moment that he removed the thermometer from the vapour, the mercury level climbed slightly . This was related to the rapid cooling (and contraction) of the glass . </P> <P> When Celsius decided to use his own temperature scale, he originally defined his scale "upside - down", i.e. he chose to set the boiling point of pure water at 0 ° C (212 ° F) and the freezing point at - 100 ° C (- 32 ° F). One year later, Frenchman Jean - Pierre Christin proposed to invert the scale with the freezing point at 0 ° C (32 ° F) and the boiling point at 100 ° C (212 ° F). He named it Centigrade (100 grades). </P> <P> Finally, Celsius proposed a method of calibrating a thermometer: </P>

When did they stop putting mercury in thermometers