LIFE AND ENERGY 13 



of it again to do work for us. Since free energy is continually 

 being converted into heat in all sorts of processes going on, it is 

 clear that the free energy of the universe is steadily decreasing. 

 This fact was pointed out by Lord Kelvin and called the " dissipa- 

 tion of energy," that is, of free energy. The energy that is lost in 

 this way has been given various names, " bound " energy, as 

 distinguished from that which is free, sometimes " entropy." The 

 last name is used when we wish to give a quantitative measure of 

 the fact, and we say that the entropy of a system is the ratio of the 

 bound energy to the absolute temperature. That energy may be 

 present, but in such a form that we cannot make use of it, may be 

 grasped by imagining that we have a hot ball of metal and a cold 

 one, so insulated from their surroundings that no heat can arrive 

 or escape. A certain amount of heat energy is present, and by an 

 appropriate mechanical device we can obtain useful work as the 

 heat passes from the hot to the cold body. But, as soon as the two 

 objects have reached the same temperature, one falling, the other 

 rising, no more work can be got, although the total amount of heat 

 present is unaltered, since none has entered or left. 



In this connection there is an important fact to be remembered, 

 a fact for which no reason can be assigned, but which is one which 

 has never been found to be otherwise. In the present universe, 

 free energy always tends to become " bound," if it is possible for it 

 to do so. If one may so express it, it takes advantage of every 

 opportunity of losing its freedom. It is not impossible to imagine 

 a state of things, otherwise similar to that which we know, where 

 free energy would tend to increase ; but it is not so as matters are 

 now arranged. 



There are three consequences of this second law which are of 

 special interest in regard to the phenomena of living organisms. 

 It has been pointed out that the so-called struggle for existence 

 is really one for the possession of free energy. There is unlimited 

 heat energy in the objects around us. What we demand is the 

 energy which is continually reaching us from the sun, and is con- 

 verted into the chemical energy of our food by the aid of the green 

 plant, as we shall find in the next chapter. The second point is 

 that, for the economical use of the energy we get from our food, it is 

 important that it should be converted into the other forms we 

 require, say that of muscular movement, without passinsLthrough the 

 stage of heat. We shall see later that appropriate mejBs are taken 

 to ensure this. The third point is that it enables j|f to predict 

 many things that happen. If we find out that a process is associ- 

 ated with a decrease of free energy, we have every reason to reckon 

 upon its taking place, whenever it can. An instance of this will be 

 seen immediately. 



