128 



GENERAL SCIENCE 



to live, we can get some vague idea of the very great 

 amount of heat in the sun. 



Measurement of Temperature. While we have no 

 difficulty in deciding whether we are too warm or too cold, 

 we cannot rely on our sense of feeling to determine tem- 

 perature except within a very narrow range ; and then the 

 result is only a comparative one. For example, it is a 

 well-known fact that if a person puts one hand into hot 

 water and the other into cold water for a time, and then 

 puts both hands into warm water it will feel cold to the 

 hand which has been in hot water, and hot to the hand 



100 C 



ing Wafer 



FIG. 111. Determination of the Fixed Points on a Thermometer. 



which has been in cold water. By comparison the sensa- 

 tions are correct, but as a test of temperature they are 

 quite unreliable. The instrument used for measuring 

 temperatures is called a thermometer. The ordinary 

 commercial thermometer consists of a capillary tube 

 with a bulb at the end. The tube is partly filled with 

 mercury, and after the air from the remaining part has 

 been removed the tube is sealed at the top. 



The freezing and boiling points of water enable us to 

 graduate the thermometer easily. The sealed tube is 

 placed in melting ice, and the point at which the mercury 



