14 INTRODUCTION TO GENERAL PHYSIOLOGY 



We may pause for a moment to point out that an explanation 

 of a phenomenon as a consequence of the laws of energetics does 

 not tell us about the mechanism by which it is effected. This 

 must be on the basis of the kinetic theory and the structure of 

 atoms and molecules. 



The statement made at the beginning of this chapter about the 

 property of living beings to produce changes may now be made 

 somewhat more precise. It is in the process of change of one form 

 of energy into another that the phenomena especially characteristic 

 of life make their appearance. When this change ceases, or, as it 

 may be put, when equilibrium has taken place, we have a state of 

 death. Just as in commerce, money that is unemployed is of no 

 value. 



The boundary surface of a liquid being in a state of tension, 

 it is clear that it may be made to do work. In a small way, the 

 experiment that we made with the soap film in a funnel shows 

 this ; the film rises and lifts up its own weight. In this case, the 

 tension in the small film is the same as that in the larger one, but 

 the area is much less, so that the energy is less. We could also 

 diminish the energy by reducing the tension without altering the 

 area. The fact reminds us that there are two factors making up 

 each kind of energy. One of these is always a sort of space or 

 mass, and is called the " capacity " factor. The other is what might 

 be called a strength or " intensity " factor. Some familiar instances 

 will make the conception clearer : 



Capacity Factor. Intensity Factor. 



Soap film - - Area. Surface tension. 



Water power - - Volume of water. Height above the earth. 



Heat - - - Quantity. Temperature. 



Electricity - - Current (ampere). Electromotive force or potential 



(volt). 



Chemical energy - Mass of material. Chemical potential. 



Instruments for measuring these factors, with the exception of 

 that of chemical potential, which is measured in a more or less 

 indirect way, are in general use. This factor of chemical potential 

 is not so easy to grasp as the others. It has been loosely called 

 "chemical affinity," but it has clearly a real existence, as may be 

 seen by the consideration that equal quantities of different com- 

 bustible materials afford very different quantities of energy when 

 burned with oxygen. And again, the chemical potential of oxygen 

 and phosphorus is high enough for combustion to take place at 

 a rapid rate, whereas oxygen and sugar only combine very slowly 

 indeed, unless we raise the potential of the oxygen. 



We may note in connection with chemical energy that there 

 can be no doubt that there is a change in the internal structure 



