Temperature. Thermometers 158 



of the cooling, remove the end of the capillary from the 

 liquid, and allow it to cool to the temperature of the 

 room. At this stage the thread of mercury should be 

 distant from the open end of the tube by about one- 

 third of the length of the tube. Should the distance 

 be less than this, heat up the tube once more for a 

 somewhat longer time than formerly, and repeat the 

 immersion in mercury and subsequent cooling to room 

 temperature. Place the tube in a large vessel of 

 water, a mercury thermometer being immersed close 

 by, and after a minute read the temperature recorded 

 by the mercury thermometer. Make a small scratch 

 with a file on the capillary tube at the point reached 

 by the lower end of the mercury thread under these 

 conditions, the tube being held in a vertical position. 

 Heat the vessel and water until the thermometer shows 

 a rise of about 20. Remove the source of heat, stir 

 well and again read the thermometer and mark the 

 tube as before. Heat up the apparatus again till a 

 further rise of 20 is shown and again mark the tube. 

 About four such determinations of the position of the 

 lower end of the mercury thread may be obtained. 

 These may be checked by allowing the vessel and its 

 contents to cool with frequent stirring of the water, 

 carefully observing whether the end of the mercury 

 column reaches the same position as previously, when 

 each of the temperatures at which the original observa- 

 tions were made is attained during the cooling. 



If we neglect any slight unevenness in the bore of 

 the tube and any small irregularity of shape at the 

 closed end, the lengths measured from the closed end 

 to the various marks made on the tube will be pro- 

 portional to the volumes of the mass of air, imprisoned 

 in the tube between the closed end and the under 



