128 



RELATION OF HEAT TO COMFORT 



the other hand, if water is heated its temperature 

 begins at once to rise, and if it is heated long enough 

 it will boil and turn to steam. Another interesting 

 thing which happens to bodies when heated is a 

 change of size. When a metal bar is heated it increases 

 in length and in volume. This is known as expansion. 

 You no doubt have observed that telephone and tele- 

 graph wires sag in the summer and are taut in the 

 winter. This again is the result of expansion. 



How do we measure heat? When we buy fuels to 

 heat our homes we should be interested in knowing 

 how much heat we get for our money. Ordinarily not 

 all of the substance in a fuel is converted into heat 

 energy. Ash may be present or there may be material 

 which escapes as unburned gas. 



to- 



30 



FIG. 199. FEVER, HOUSEHOLD, AND 

 CANDY THERMOMETERS 



Since heat is a form of energy and not a substance, 

 it cannot be measured directly in pounds or grams. 

 It must be measured by the effects it produces. The 

 unit of measurement of the heat value of fuels in this 

 country is the "British thermal unit"; it is written 

 B. T. U. It is the amount of heat required to raise one 

 pound of water one degree Fahrenheit. The heat 

 values of our common fuels, expressed in Calories per 

 pound and British thermal units per pound, are given 

 below. 



CALORIES PER POUND 



B. T. U. PER POUND 



In European countries and in our own scientific lab- 

 oratories the Calorie is the unit of measurement of 

 heat. A Calorie is the amount of heat required to raise 

 one kilogram of water one degree centigrade. This 

 means that a kilogram of water rising one degree cen- 

 tigrade takes in one Calorie of heat, while a kilogram 

 of water cooling one degree centigrade loses one Cal- 

 orie of heat. 



Do substances vary in their ability to absorb heat? 

 It is an interesting fact that various materials differ 

 greatly in their capacity to absorb heat. A kettle of 

 water on a stove will not heat as quickly as a piece of 



c 



xx 



BOILING POINT OF WATER 



Htfel 



-lit..... NORMAL BODY TEMPERATURE 



FREEZING POINT 

 OF WATER 



FIG. 200 



iron of the same weight. Experiments show that wa- 

 ter, per unit of weight, requires more heat to raise its 

 temperature one degree than any other common sub- 

 stance. Also it cools more slowly than other sub- 

 stances because it has more heat to lose. The amount 

 of heat that will raise one gram of water one degree 

 centigrade will raise one gram of iron about ten de- 

 grees centigrade, one gram of silver twenty degrees 

 centigrade, and one gram of lead about thirty-three 

 degrees centigrade. 



REFERENCES FOR FURTHER STUDY 

 Texts 



Caldwell and Curtis, Science for Today, Chap. 12 

 Clement, Collister, and Thurston, Our Surroundings, Chaps. 



7, 8 

 Hunter and Whitman, My Own Science Problems, Unit 5 ; 



Science in Our Social Life, Unit 5 

 Lake, Harley, and Welton, Exploring the World of Science, 



Chap. 20 

 Pieper and Beauchamp, Everyday Problems in Science, Unit 



10 

 Powers, Neuner, and Bruner, The World around Us, Chap. 



20; Man's Control of His Environment, Chap. 27 

 Skilling, Tours through the World of Science, Tour 14 



