MEASUREMENTS OF HEAT 133 



when its temperature falls i C.) is called the specific 

 heat of that substance. You have already learned that some 

 substances "heat up" more rapidly than others, which 

 goes to show the fact that substances vary as to their 

 specific heat. The lower the specific heat, the more quickly 

 a substance heats up. Thus mercury, whose specific 

 heat is low, heats up thirty times as quickly as water; 

 that is, to raise i cc. of mercury i C. requires the applica- 

 tion of only one-thirtieth of a calory. 



With the exception of hydrogen, water has the highest 

 specific heat of any known substance. Since, therefore, 

 water absorbs more heat than other substances as its tem- 

 perature rises, it follows that it gives out more heat as its 

 temperature falls. So we perceive why water is so useful 

 in heaters, in hot- water bags, or in foot- warmers; no other 

 substance could be used which would give out so much heat. 

 Have you ever noticed how much more likely you are to 

 burn your mouth with hot chocolate than with hot coffee or 

 tea? What is the explanation of this? 



The calorimeter, which measures the number of calories 

 in substances, does this by measuring the amount of 

 heat which they give off to a given amount of water. 

 There are various types of calorimeters. The simplest 

 is a metal vessel covered with some non-conductor (usually 

 felt) to prevent the loss or gain of heat from the surround- 

 ing air. In this vessel there is a quantity of water of known 

 volume and temperature. The substance to be tested 

 is put into the water and the change produced in the tem- 

 perature of the water carefully noted. From this can 

 be estimated the number of calories which have been added 

 or subtracted by the substance introduced. 



