A mercury-in-glass thermometer is a thermometer consisting of mercury, in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the tube, which varies according to the temperature. To increase the sensitivity, there is usually a bulb of mercury at the end of the thermometer which contains most of the mercury; expansion and contraction of this volume of mercury is then amplified in the much narrower bore of the tube.
Anders Celsius devised the Celsius scale, which was described in his publication the origin of the Celsius temperature scale in 1742.
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 melting temperature and not the freezing temperature. 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 vapor (when this is done to high precision, a variation will be seen with atmospheric pressure). At the moment that he removed the thermometer from the vapor, the mercury level climbed slightly. This was related to the rapid cooling (and contraction) of the glass.
The air pressure influences the boiling point of water. Celsius claimed that the level of the mercury in boiling water is proportional to the height of the barometer.
When Celsius decided to use his own temperature scale, he chose to set the boiling point of pure water at 0°C and the freezing point at 100°C. One year later Frenchman Jean Pierre Cristin proposed an inverted version of the scale (freezing point 0°, boiling point 100°). He named it Centigrade.
Finally, Celsius proposed a method of calibrating a thermometer:
Place the cylinder of the thermometer in melting pure water and mark the point where the fluid in the thermometer stabilizes. This point is the freeze/thaw point of water. In the same manner mark the point where the fluid stabilizes when the thermometer is placed in boiling water vapor. Divide the length between the two marks into 100 equal pieces.
These points are adequate for approximate calibration but both vary with atmospheric pressure. Nowadays, the triple point of water is used instead (the triple point occurs at 273.16°kelvin, 0.01°C).
Since then, mercury thermometers have become increasingly rare, as mercury is highly and permanently toxic to the nervous system. Many countries have banned them outright from medical use.