CHAPTER X 

 HEAT AND CHANGE OF STATE 



57. The Calorie. The temperature of a body is not neces- 

 sarily an indication of the amount of heat it contains. A small 

 pool on a summer day may be warmer than the nearest lake, 

 and yet anybody can understand that the lake must have a 

 much larger amount of heat in it. The thermometer merely 

 indicates the temperature of a body. To measure the total 

 amount of heat in it, we need a new standard of comparison. 

 Such a standard is found in the calorie, which is defined as the 

 amount of heat necessary to raise the temperature of one gram 

 of water one degree Centigrade. Since all substances give off 

 as much heat in cooling as they took up in warming, our calorie 

 could as well have been defined as the amount of heat that 

 must be withdrawn to lower the temperature of one gram of 

 water one degree Centigrade. Another unit of heat some- 

 times called the large calorie is a thousand times larger than 

 the calorie we have discussed. This latter, however, is more 

 properly called the kilocalorie. 



58. Specific Heat. When we add equal amounts of heat to 

 equal weights of different substances, we discover that all do 

 not increase in temperature at the same rate; that is, it takes 

 more heat to raise a gram of some substances one degree in 

 temperature than it does others. The amount of heat that 

 will raise a kilogram of water one degree Centigrade will raise 

 an equal weight of copper ten degrees, of silver or tin twenty 

 degrees, and of mercury thirty degrees. Land heats up four 

 times as fast as water. The amount of heat required to raise 

 the temperature of any substance one degree in comparison 



60 



