HEAT. 49 



it would be sufficient to raise. 420 kilograms 1 meter high, or 3000 pounds 1 foot 

 high L e., the calorie is equivalent to 420 kilogram-meters, or 3000 foot pounds. 

 The heat generated by combustion is determined in the laboratory by means of 

 an apparatus known as the calorimeter. This is generally so constructed that 

 a definite weight of substance may be burned in a chamber surrounded by cold 

 water. The rise in temperature of this known quantity of water serves as the 

 basis for calculation. 



Specific heat. Equal weights of different substances require dif- 

 ferent quantities of heat to raise them to the same temperature. For 

 instance : The same quantity of heat which is sufficient to raise 1 

 pound of water from 60 to 70 will raise the temperature of 1 

 pound of olive oil from 60 to 80, or that of 2 pounds of olive oil from 

 60 to 70. Olive oil consequently requires only one-half of the heat 

 necessary to raise an equal weight of water the same number of degrees. 

 As water has been selected as the standard for comparison, we may 

 say that specific heat is the heat required to raise a certain weight of 

 a substance a certain number of degrees, compared with the heat 

 required to raise an equal weight of water the same number of 

 degrees. 



The heat required to raise 1 gramme of water 1 degree centigrade 

 is usually taken as the unit of comparison. On thus comparing 

 olive oil, we find its specific heat to be J. If we say the specific 

 heat of mercury is ^-, we indicate that equal quantities of heat will 

 be required to raise 1 pound of water or 32 pounds of mercury 1 

 degree, or that the heat which raises 1 pound of water 1 degree will 

 raise 1 pound of mercury 32 degrees. 



Conduction of heat. When the end of a metallic bar is heated, 

 a rise in its temperature is soon noticed at a distance from the heated 

 part. This transfer of heat from some source, for instance from a 

 flame to a cold substance, is practically a transfer of motion from 

 more rapidly moving molecules to those moving more slowly. It 

 may be compared to the motion imparted to a billiard ball at rest or 

 moving slowly by another ball propelled with greater velocity. The 

 more rapidly moving ball will lose part of its velocity in imparting 

 motion to the other ball. Similarly the rapidly moving molecules of 

 the hot body in transferring motion to molecules moving slowly in 

 the cold body lose some of their velocity i. e., the hot body itself 

 becomes cooler. The expression " cooling off" must never be under- 

 stood to imply a transfer of cold, but always a removal of heat. In 

 taking a cold bath, or in applying an ice-bag to a fever patient, we 



