760 THE THERMOMETER. 



The instruments for measuring temperatures called 

 c thermometers ' depend on the expansion of liquids by 

 heat. Solid bodies are not suited for thermometers, 

 because their expansion is too small, and therefore the 

 change of volume, due to a given change of tempera- 

 ture, cannot be easily observed. Gaseous bodies, on 

 the other hand, are unsuitable for thermometers, espe- 

 cially those for common use, partly because they expand 

 too much, partly because their volume depends not 

 only on their temperature but also on the pressure of 

 the atmosphere, which, as we know, is constantly vary- 

 ing. In far the greater number of thermometers, 

 mercury is the liquid used. This liquid is preferable 

 to most others, on account of various properties 

 which mercury possesses. It very soon takes the 

 temperature of surrounding bodies, because heat passes 

 very easily in and out of mercury, and hence it indicates 

 rapidly whatever changes of temperature take place ; it 

 also expands regularly in proportion to the temperature, 

 that is, a double or treble increase of temperature causes 

 a double or treble expansion, and so on ; its opacity 

 allows of its being easily seen when contained in very 

 narrow glass tubes ; it does not wet glass, hence, when 

 the surface of mercury falls in a glass tube, nothing is 

 left adhering to the sides of the tube ; finally, it is capable 

 of passing through a long range of temperature without 

 changing its state of aggregation, for it boils at a 

 temperature which is higher than the fusing point of 

 lead, while it does not freeze in the greatest winter cold 

 observable in Central and Western Europe. At a very 

 low temperature, such as may be observed in higher 



