HUMAN STRUCTURE AND LIFE-ACTIVITIES 493 



may be changed into mechanical energy by a steam-engine ; 

 and this into electrical energy by a dynamo ; and the elec- 

 trical energy into heat energy by an electric-stove, or into 

 light by an electric-lamp, or into mechanical energy by a 

 motor. Thus the stored energy of coal may be changed into 

 various other forms of energy which are known as mechanical 

 energy, heat, light, and electricity. 



One of the important discoveries of the last century was 

 that in such changes no energy is lost to the world. At first 

 this statement will be puzzling to one who has read that 

 85 per cent of the energy stored in a ton of coal does not 

 appear in the work of the best steam-engines in common use, 

 and hence seems to be lost energy. Also, there appears to 

 be a loss in transforming to electricity, for a 20-horse-power 

 steam-engine driving a dynamo cannot generate enough 

 electricity to drive a 20-horse-power electric-motor. How- 

 ever, this loss of energy from the practical standpoint is, 

 after all, not a real loss, for the same amount of energy 

 remains in the universe. This could be proved by placing 

 a small steam-boiler and its furnace, an engine, a dynamo, 

 and a motor together in a room with walls which would 

 prevent loss of heat. Then if some fuel were burned and 

 the machinery set to work, generating steam and electricity, 

 the explanation of the apparent loss of energy would be 

 found chiefly in the heat given off to the air in the room. 

 Adding this to the energy of the engine, dynamo, and motor, 

 the sum would equal the amount of energy which a chemist 

 can demonstrate by burning the same amount of coal in a 

 calorimeter ( 418). In short, from 85 to 90 per cent of the 

 energy stored in coal is given off from the machinery as heat, 

 10 to 15 per cent appears in the mechanical energy of the 

 steam-engine, some less in the electrical energy of the dynamo, 

 still less in the energy manifested in the electric-lamp, heater, 

 or motor. But each apparent loss is represented by heat 

 given off to the surrounding air. The fact is, then, that 



