MECHANICAL ENERGY AND HEAT 189 



through the agency of the coal which is put into the furnace. 

 Since the origin of foods can be traced back to the manufacture 

 of carbohydrates through the aid of the sun's energy (Chap- 

 ter VII), it is not difficult to see that the sun is the ultimate 

 source of energy for all the activities of living things. This 

 matter will be taken up again in connection with our study 

 of the nutrition of plants and animals. 



