268 THE REALITIES OF MODERN SCIENCE 



temperature will be the lowest possible. The ratio 

 of all the energy in the universe to its absolute tem- 

 perature will then be as large as it can become. Now 

 this quotient of heat energy and absolute temperature 

 is usually called " entropy." The natural processes 

 or transformations of the universe tend to increase 

 the entropy. 



The processes mentioned above are natural much as 

 it is natural for water to run down hill. We can make 

 water go up hill, but it is not natural and we have to do 

 work. In the same way we can separate the molecules 

 of gases which have mixed by diffusion, we can produce 

 work from heat, and we can transfer energy from cold 

 bodies to hot ones, making the former colder and the 

 latter hotter, but these are not natural processes. In 

 every instance we have to do work to accomplish the 

 result. We can only do the necessary work at the 

 expense of some other energy transformation which 

 is natural. Those processes, which are the reverse 

 of the natural ones, have never been known to occur of 

 their own accord and scientists are agreed in believing 

 that they never will. They may be produced, but 

 only when a transformation which is natural supplies 

 the energy. 



These statements are the sense of the Second Law of 

 Thermodynamics. The mathematical statement of the 

 law is usually made in terms of that ratio of molecular 

 kinetic energy or heat to absolute temperature which 

 we have just called " entropy." The law says that 

 all natural transformations in the universe result in an 

 increase in entropy. The student of thermodynamics, 

 however, finds it a great convenience to reason about 



