296 BIOLOGY 



much smaller particles called atoms, which are quite invisible 

 even with the highest-power microscope. They also tell us 

 that these atoms are united in groups, which are called mole- 

 cules, each consisting of a number of atoms. Just as it re- 

 quires the expenditure of energy to lift stones into position 

 to form a monument, it also requires energy to lift atoms 

 into position to form a molecule; and if the molecule is 

 broken down, the energy is liberated according to the same 

 principle concerned in liberating it when a monument falls. 

 If, therefore, we look upon the particle of coal as a series of 

 molecules, each built up of many atoms, it follows that if these 

 tiny molecules are broken down, so that their atoms will assume 

 a simpler form, the energy imprisoned in them, in a dormant 

 state, will be released. Coal is thus made of immense numbers 

 of complex molecules, each of which has been built by the 

 expenditure of energy, and the coal contains, in a potential 

 form, energy which may be released by breaking up the coal. 

 The molecule is broken down when the coal is burned and its 

 energy appears in the form of heat, which may then be applied 

 to the moving of an engine. This of course raises the question 

 as to how the energy was stored away in the coal, a question 

 to which we will refer later. 



THE TRANSFORMATION OF ENERGY 



Any type of energy may be converted into any other type. 

 When we lift a stone to the roof of the house we convert energy 

 of motion into energy of position, and when the stone falls, 

 energy of position is converted again into energy of motion. 

 When it is halfway to the ground, it has a certain amount of 

 energy of motion, because it is moving; but it also has a cer- 

 tain amount of energy of position, because it is considerably 

 above the surface of the earth. The more closely it approaches 

 the earth, however, the more its energy of position is converted 

 into energy of motion, and the moment it strikes the ground, 

 all of its energy of motion is converted into heat. The potential 



