34 ORIGIN AND NATURE OF LIFE 



create matter. Thus the position is arrived 

 at that instead of matter and energy being 

 fixed in amount, and always conserved, they 

 are mutually convertible into each other, as 

 soon as we have discovered the appropriate 

 mechanisms for the purpose- 

 Again, energy is usually divided into two 

 main varieties, each of which possesses several 

 forms, these varieties are termed kinetic 

 energy and potential energy. Kinetic energy 

 is energy due to motion, either of a large body 

 as a whole, or of the atoms or molecules, which 

 form the minute mechanically indivisible 

 portions of matter, while potential energy 

 is energy due to the position of portions of 

 matter in relationship to one another, and 

 here again the matter may be in larger masses, 

 or may consist of molecules or atoms. Ex- 

 amples of kinetic energy are such as (a) large 

 bodies in motion ; (b) heat ; (c) light ; (d) 

 electricity ; (e) radiations of many kinds ; 

 (/) the molecular motions of a gas, liquid, or 

 solid ; and (g) the motions of constituent 

 atoms of a molecule in relationship to one 

 another. As examples of potential energy (in 

 which motion does not come in) there may be 

 quoted (a) bodies raised above the earth's 

 surface, either solid, or fluid, like water in a re- 

 servoir, or gaseous, like barometric differences 



