62 THE PHILOSOPHY OF BIOLOGY 



containing available energy. Let the mixture be 

 fired by an electric spark, and heat is evolved. The 

 total energy of the system is unaltered in amount, but 

 the available energy has disappeared, since the heated 

 water vapour is incapable of undergoing further trans- 

 formations while it forms part of its isolated system. 1 



All physical processes are therefore irreversible, 

 that is to say, proceed in one direction only. Either 

 a process is irreversible in the sense that it cannot 

 proceed both in the positive and negative directions 

 (a steam-engine, for instance), or it is irreversible 

 in the sense that while it proceeds the energy in- 

 volved in it becomes less capable of being transformed 

 into other conditions. (In the theoretically reversible 

 dynamo, energy becomes dissipated in the form of 

 heat.) The following statements may be regarded 

 as axioms 2 : — 



(i) " If a system can undergo an irreversible change, 

 it will do so." 



(2) "A perfectly reversible change cannot take 

 place by itself." 



In the phenomena studied by physics we see only 



1 It is really necessary to lay stress on the distinction between available 

 and unavailable energy, as it is one which many biologists appear to ignore. 

 Thus, a popular book on the making of the earth attempts to argue that 

 essential distinctions between living and inorganic matter are non-existent. 

 One of these distinctions is that organisms absorb energy, and this author 

 points to the absorption of " latent heat " by melting ice as an example of 

 the absorption of energy in a purely physical process. Consider a system 

 consisting of a block of ice and a small steam boiler. We can obtain work 

 from this by the melting of the ice — that is, its " absorption of latent heat." 

 The system, ice at o° C. + steam at 100° C, possesses available energy, but the 

 system, melted ice + condensed steam, both at the same temperature, contains 

 none. The molecules of water at o°C, " absorb energy," that is to say, their 

 kinetic energy becomes greater, but their available energy in the system has 

 disappeared. In saying that the organism absorbs energy, we mean, of 

 course, that it accumulates available energy, that is, the power of producing 

 physical transformations. (See further, appendix, p. 366.) 



2 Bryan, Thermodynamics: Teubner, Leipzig, 1907, p. 40. 



