28 A YEAR IN SCIENCE 



thermometer is divided into one hundred equal divisions 

 between the freezing and boiling points of water. The 

 freezing point of water is marked and the boiling 

 point of water 100. 



Construction of centigrade thermometer. From your 

 examination of a centigrade thermometer you have 

 discovered that it consists of a hard glass tube of vary- 

 ing length, with a very fine hair-like opening or bore 

 running through the center. One end of the tube is 

 blown out into a small bulb, filled with mercury, which 

 also projects into the hair-like bore. In some thermom- 

 eters colored alcohol is used instead of mercury. 



The other end of the tube is closed and the stem of 

 the tube is marked for degrees, with the ' marked 

 freezing and the 100 marked boiling. The height to 

 which the mercury rises is called the temperature of 

 the given substance. Thus if mercury in a centigrade 

 thermometer is at 20 in the room in which you are 

 sitting, then we say the temperature of the room is 

 20C. 



Determination of freezing 1 and boiling points. To 

 construct a thermometer and determine the essential 

 points, a glass tube of fine bore is taken and the bulb 

 and tube are filled with mercury at a temperature 

 slightly above the highest temperature for which the 

 thermometer is to be used. The open end of the tube 

 is then sealed off in a hot flame. As the mercury cools 

 and withdraws from the tube into the bulb, it leaves a 

 vacuum above it. 



