STUDIES IN NATURE 



to tell us what is the actual degree of heat or 

 the temperature of the objects around us. There 

 is a thermometer somewhere in most houses, in 

 schoolrooms, in chemist's shops ; and if we have 

 not seen one already, we ought not to have much 

 difficulty in getting someone to show us one. 

 There are many different sorts of thermometers, 

 depending on the purposes for which they are 

 required, but I will describe a very common 

 kind which we are all sure to come across sooner 

 or later. It has a fine glass tube, closed at the 

 top end so that nothing can get in ; at the bottom 

 there is a small glass bulb which is filled with 

 the grey liquid metal called mercury, and this 

 goes partly up the tube. The glass tube has 

 sometimes little marks or scratches on it by 

 which we can measure how far the mercury has 

 risen in the tube. Sometimes the glass tube 

 and bulb are fixed on to a frame of wood, and 

 the measuring marks are put on to that instead. 

 The liquid metal mercury, which we mentioned 

 in our last chapter, like the other metals we 

 have just been speaking about, takes in and 

 gives out heat very readily, and quickly becomes 

 of the same temperature as the things around 

 it. It has another useful property. Like water 

 and air and most other substances, it expands 

 and takes up more room when it is hot than 

 when it is cold. The mercury in the glass bulb 

 cannot expand there, for it has no room, as a 

 sort of glass is chosen to make the bulb and 



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