IS THE ORGANISM A MECHANISM? 649 



matter (in modem views the latter is, of course, contained in the 

 former), we arrive at the conclusion that nothing in the universe 

 can be created, nor can anything be destroyed. But the second 

 law of thermodynamics states that something which is char- 

 acteristic of an isolated physico-chemical system, its sum of 

 entropy, tends continually to become augmented. Our only 

 isolated system is the universe, and the most general statement 

 of the second law is that the sum of entropy of the universe 

 tends continually to a maximum. 



We cannot discuss here the shadowy mathematical concept 

 called entropy, but we may state the second law (partially, but 

 correctly for our purpose) in saying that something is irretriev- 

 ably destroyed in every physico-chemical reaction that occurs. 

 This something is available energy. Energy, potential or 

 kinetic, that is, energy of position or the energy of motion of 

 entities possessing mass, which can be made use of in producing 

 or setting up transformations, or natural phenomena, is available 

 energy. The heat energy of one part of a system which is at 

 a higher temperature than another part, for instance, the energy 

 of steam in a boiler in relation to the energy of the condenser 

 water is available. Energy which cannot be utilised to set 

 up transformations — to do work — is unavailable : such is, for 

 instance, the heat energy of the ocean in relation to the engines 

 of a ship traversing it. In order that some of the energy of a 

 system may be available some part of it must be at a higher 

 potential than another part. If there are no differences of potential 

 the energy is unavailable. Now, in all natural changes, or 

 phenomena, or energy-transformations, there is an ever-present 

 tendency for some fraction of the total energy manifested in the 

 change to become converted into low-temperature heat. Such 

 heat becomes conducted or radiated into the earth or into space, 

 becomes uniformly diffused, and so becomes unavailable. This 

 occurs in endothermic and exothermic processes. In the imagi- 

 nary world of mathematical physics natural phenomena may 

 occur in systems with perfectly elastic or perfectly rigid parts, 

 where there is no friction, and where heat is either perfectly 

 conducted or perfectly insulated. But in the real world all 

 natural processes are such as involve friction and loss of heat. 

 In all of them some energy becomes unavailable. All of them 

 are irreversible processes, that is, processes which cannot be 

 retraced. With every such irreversible process some part of the 



