CONSERVATION OF ENERGY 319 



the plant 's tissues. This energy is furnished by the 

 sun in the form of light and heat. The same amount of 

 energy which is given by the sun to the plant is again 

 given out quickly and intensely if the plant is burned, 

 or slowly and imperceptibly if the plant decays. Dur- 

 ing the growth of the plant, the energy is being stored 

 up. The storing up of energy in this way so that it 

 is not lost, although for a longer or shorter time it may 

 seem to disappear, is an example of the natural law 

 known as the conservation of energy. 1 



Transformation of energy. Energy, like matter, can 

 never be destroyed. If, for example, the wood from a 

 tree, which has been built up of carbon and hydrogen ob- 

 tained by the use of the sun's energy, is burned under a 

 boiler, the energy set free by the burning of the wood 

 causes the little particles of water in the boiler to take on 

 so violent a motion that in their effort to escape from 

 confinement they press upon the piston of the engine 

 and drive it vigorously back and forth. As a result,, 

 an entirely different form of energy, namely, mechanical 

 energy, is developed. The engine may in turn run a 

 sawmill, a piledriver, or a dynamo for the generation 

 of electricity. In any case, there is an exact equality, 8 

 or correlation, between the original heat from the sun, 

 the force developed in the engine and the work done 



1 Similarly, the labor (energy) expended in raising building 

 materials to their positions in a building, although seemingly lost 

 for many years, again becomes active when the materials ulti- 

 mately fall to the earth. 



2 All of the sun's heat is not made productive, since a part of it 

 escapes as hot gases up the chimney, a part is radiated from boiler 

 and engine, and a part is neutralized by the friction in the bear- 

 ings of the engine and machinery run thereby. The losses in the 

 ordinary boiler and steam engine amount to about 85 per cent of 

 the heat generated by the burning fuel. Thus only 15 per cent of 

 the energy developed from the combustion of the fuel is utilized in 

 useful work. 



