78 GENERAL SCIENCE 



other substance, except hydrogen gas (which is not con- 

 venient to work with), the amount of heat necessary to 

 change the temperature of i gram of water i C. was taken* 

 as the unit of heat the calorie. All ordinary substances 

 require less than a calorie of heat to change the tempera- 

 ture of i gram i C. If a pound of iron and a pound of 

 water (a pint) are placed on a hot stove or a gas fire, 

 which will get too hot to hold in the hand, the quicker? 

 The temperature of the iron will increase about nine 

 times as fast as the temperature of the water. The 

 temperature of a pound of mercury will increase 30 times 

 as fast as that of a pound of water under the same condi- 

 tions. That is, it takes 30 times as much heat to change 

 the temperature of one gram of water i C. as it does to 

 change the temperature of one gram of mercury i C. 



Because of this variation in the amount of heat neces- 

 sary to change the temperature of a gram of any substance 

 i C. we have what is called specific heat. The specific 

 heat of a substance is the amount of heat needed to change 

 the temperature of one gram of that substance i C., or it is 

 the amount of heat given out when the temperature of 

 one gram of that substance falls i C. in temperature. 

 The specific heat of water is i, of iron .11, of mercury 

 .033, of copper .095, of lead .031. That is, it takes .11 

 of a calorie to change the temperature of i gram of iron 

 i C., and .033 of a calorie to change the temperature of 

 i gram of mercury i C. 



58. Sources of Heat. The chief natural source of 

 the earth's heat is the sun. The rays of the sun warm 

 the surface of the earth, and it in turn warms the air. 

 The air does not absorb much heat directly from the 

 sun's rays. It is the sun's heat that makes our summers 

 warm and gives energy to the growing vegetation upon 



