60 HEAT. 



sufficient cold it reaches 39 degrees below zero. 

 There, intense cold has no effect upon it, for at this 

 point the mercury freezes. How much colder it is 

 than 39 cannot be told, therefore, by the mercurial 

 thermometer. Thermometers containing alcohol in- 

 stead of mercury are used for this purpose, because al- 

 cohol never freezes, and will continue to sink further 

 and further in the tube the colder it grows. 



133. EXTREME HEAT, HOW MEASURED. 



How is extreme 



heat meas- If a Fahrenheit thermometer is heated, 

 the mercury in it rises, till it reaches 662, 

 and then begins to boil. A little more heat forms suf- 

 ficient vapor of mercury to burst the tube. For this 

 reason, a mercurial thermometer cannot be used to 

 measure extreme heat. A platinum bar inclosed in a 

 black lead tube shut at the bottom, is common- 

 ly employed for this purpose. Tube and bar are 

 placed on the fire, or in the melted metal, whose 

 heat it is desired to measure, one end being left 

 out, so that it can be seen. The consequence is 

 that the platinum bar expands, and projects 

 from the earthen tube. The tube itself expands but 

 little. The further the bar projects, the greater is the 

 heat. As it pushes out, it is made to move an index 

 hand, and point to the number indicating the tempera- 

 ture, on a graduated arc. This arc is first graduated by 

 repeated trials, observing how much the bar projects 

 and moves the hand by the same heat which raises 

 the mercury one degree in the Fahrenheit thermometer. 



n ., ., 134. THE AIR THERMOMETER. A col- 



Describe the , 



air thermome.- umn of air confined in a glass tube over 

 ***' colored water, was the first thermometer 



