HEAT 77 



two rooms do not tell you how much heat there is in each, 

 but only the comparative degree of heat. The tempera- 

 ture does help in finding the amount of heat in the water. 

 The quantity of heat put into a gallon of water cannot 

 be determined unless the temperature is known when 

 the heat is applied and also when the water is removed 

 from the fire. But a degree of heat on the thermometer is 

 not a unit of heat, it is a unit of temperature. So we 

 need to make use of another unit, the unit of heat. 



56. Calorie. The calorie is the unit of heat. The 

 calorie is the amount of heat necessary to change the tempera- 

 ture of one gram of water i C., or it is the quantity of 

 heat given out by one gram of water when it cools i C. 

 If 500 grams of water are heated from 5 C. to 25 C., the 

 change in temperature is 20 C.; 500X20 = 10,000, the 

 number of calories of heat taken up by the water. If 

 1,000 grams coolfrom 40 to 25, the amount of heat 

 given out is 15X1000, or 15,000 calories. 



To measure the length of your desk you would use 

 inches, to measure the length of the room you would 

 use feet or yards, to measure across a state you would use 

 miles. All are units of length but of different size. The 

 reason for using large units to measure large quantities 

 is to avoid such large numbers. So for measuring a 

 large amount of heat we have the great calorie or kilocalorie. 

 It is 1,000 times as large as the calorie, defined above. 

 The great calorie is used in measuring the heat we get 

 from foods and also the fuel value of foods for animals. 

 In tables giving the fuel value of the various kinds of 

 foods, the calorie used is the great calorie. 



57. Specific Heat. Since it requires more heat to 

 change the temperature of a gram of water i C. than 

 to make the same change in the same weight of any 



