ENERGY AND CHEMICAL CHANGE 163 



or absorb, according as the action is exothermal or endothermal, 

 amounts of heat which are equal. 



The rate at which a given chemical action is allowed to take place 

 has no influence on the total amount of heat ccnsumed or pro- 

 duced. It may not at first sight appear obvious that rusting 

 evolves heat, but a delicate thermometer will show that a heap of 

 rusting nails is somewhat higher in temperature than surrounding 

 bodies. Poor conductors, like oily rags and ill-dried hay, show a 

 tendency to spontaneous combustion owing to accumulation of the 

 slowly developing heat of oxidation (p. 41). The warmth of our 

 own bodies is due to the same cause. 



It should be noted that production or absorption of heat is not, 

 in itself, an evidence of chemical action. Physical changes are all 

 likewise accompanied by the same phenomena. Thus, the evapo- 

 ration of water absorbs heat, and condensation of a vapor and the 

 crystallization of a supercooled liquid liberate heat. 



Exercises. 1. Which form of energy is delivered as such, 

 and paid for as such, in most cities? 



2. How many calories are required to raise 500 g. of a sub- 

 stance of specific heat 0.5 from 15 to 37? 



3. The combustion of 1 g. of sulphur to sulphur dioxide de- 

 velops 2220 calories. What is the heat of combustion of sulphur? 

 Write the thermochemical equation. 



