3 o PRINCIPLES OF GENERAL PHYSIOLOGY 



It may assist in understanding the meaning of chemical potential if we remember that, in 

 a voltaic cell, chemical energy is directly converted quantitatively into electrical energy. 

 Faraday showed that the quantity of electricity obtained is propoi tional to the amount of 

 chemical change, so that the capacity factors of the two kinds of energy are proportional. 

 Hence the intensity factors are also proportional, or electromotive force is a measure of 

 chemical affinity. Faraday, therefore, was justified in regarding electrical force and chemical 

 affinity as one and the same, as Mellor (1904, p. 26) points out. 



Ostwald (1900, i. p. 249) regards chemical energy as being of as many kinds 

 as there are elements ("Stoffe"). We have seen already how the intensity 

 factor of energy in general never increases of itself; so that if the chemical 

 potential of the products of a given reaction is higher than that of the reacting 

 bodies, that is, when a substance is produced requiring to be supplied with 

 energy, an endothermic reaction in fact, energy must be supplied from some 

 extraneous source : it may be heat from neighbouring bodies or chemical energy 

 from a concurrent reaction, involving fall of potential, in the same system. In 

 the last case we have what is known as a " coupled reaction." 



While, therefore, there is only one kind of temperature, or two kinds of 

 electromotive force, positive and negative, which can be increased or diminished 

 by altering the magnitude of the forces producing them, chemical potential 

 cannot be increased directly by the fall of potential in another reaction with 

 dissimilar components. 



Ostwald gives the following example : Hydrogen peroxide is a body of higher potential 

 than water or oxygen. Hence, in order to form it, the potential of oxygen must be raised, 

 or the oxygen made "active." This cannot be done by smy or every kind of reaction pro- 

 viding energy in the system, the neutralisation of acid, for example, but must come from 

 a reaction such as the oxidation of phosphorus, in which part of the oxygen taking part in 

 the reaction is made active by means of energy derived from the other part of the 

 reaction in which the potential of phosphorus is lowered by conversion to oxide. 



The expression for the maximal work (A) of a chemical process is given by 

 Nernst (1911, p. 658) as 



A = RTlog,K, 



where R is the gas constant, T absolute temperature, and K the equilibrium 

 constant of a reversible reaction. All reactions can be treated as reversible. 

 As it is put by J. J. Thomson (1888, p. 281), if we were able "to control the 

 phenomenon in all its details, it would be reversible, so that, as was pointed out 

 by Maxwell, the apparent irreversibility of any system is due to the limitation 

 of our powers of manipulation." K, in the above formula, may be regarded as 

 the ratio of two opposite reactions. It follows at once that the greater K is, 

 that is, the nearer to completion the reaction proceeds in one direction, the 

 greater the amount of energy available. In some cases we know the value of 

 K, so that the free energy of the reaction can be calculated at once. 



Nernst (1911, pp. 709-716, and 1913, pp. 741-753) has also put forward a new method which 

 he thinks may lead to the determination of the free energy of any chemical reaction. Limits 

 of space forbid its description here, and readers interested may consult the original (see also 

 the work of Pollitzer, 1912). 



It is held by Wegscheider (1912, pp. 223-238) that the maximal work to be obtained 

 consists of two parts, one which is only to be got by making it to overcome external pressure, 

 and is zero at constant volume; the other can be obtained in other ways, as electromotive 

 force, for example. He gives formula; for the minimum total work, for the electromotive 

 force of chemical reactions, the dissociation of a gas, and a reversible gas battery. 



The monograph by Helm (1894) may be consulted with profit. 



SURFACE ENERGY 



We shall see in the next chapter how the surface of contact of a liquid 

 with a solid, a gas, or another liquid, with which it does not mix, the interface 

 between any heterogeneous phases, in general, has the properties of a stretched 

 film. It can therefore do work when this tension is able to decrease. Now if 

 we consider the energy available in a living cell, we see that, although chemical 

 potential can exert its full effect in a small space, the capacity factor of 

 chemical energy needs considerable active masses in order that much total 

 energy shall be afforded. In surface energy, on the other hand, although the 



