JOSIAH WILLABD GIBBS 47 



being functions of the temperature and the chemical potentials. The 

 only stable substance which can be formed between two other phases 

 will be the one having the least surface tension. 116 The chemical equi- 

 librium of solids in contact with liquids, including the delicate mathe- 

 matical conditions for the formation of crystals in mother liquor, is 

 treated dynamically as a matter of stresses and strains, and this together 

 with the theory of interfacial formations and liquid films will embrace 

 the possible physics of colloid substances. Gibbs gives for the first 

 time a mathematical discussion of the mode of formation of liquid 

 films and the conditions for their stability and his dynamic explana- 

 tion of the black spots on soap films 117 was proved quantitatively in 

 1887 by Eeinold and Eiicker's micrometric data of the relations be- 

 tween the thickness and surface tension of these films. 118 The impor- 

 tance of liquid films in biology is obvious, and this phase of Gibbs's 

 theory, which is capable of the widest development, has as yet received 

 the slightest attention. 



Electrochemical Thermodynamics. — One of the most important 

 features of energetics is Gibbs's theory of the galvanic cell which shows 

 the close interrelation existing between chemical, thermal and electric 

 energy. The earliest pioneer in this field was Lord Kelvin, and, prior 

 to 1878, physicists had accepted the Joule-Kelvin theory that the electro- 

 motive force of a galvanic apparatus is the mechanical equivalent of 

 the total chemical energy liberated per unit strength in unit time. But 

 this view, which implies that all the electric energy of a chemical cell 

 is available, did not agree entirely with the experimental data of Boscha, 

 Eaoult and others. It was corrected and modified by Gibbs, who 

 showed that the electromotive force of the cell is in reality its free 

 energy per electrochemical equivalent of decomposition, 119 from which 

 it follows that neither solidification nor fusion of the metals at the 

 temperature of liquefaction should cause any abrupt alteration of the 

 electromotive force. In 1882, six years later, this important theorem 

 was rediscovered from a different view-point by Helmholtz and bril- 

 liantly developed as to experimental confirmation. 120 The Gibbs Helm- 

 holtz doctrine enables the physicist to trace out the variations in electro- 

 motive force due to chemical differences in different cells. In a letter 

 to Professor Bancroft; now printed in the memorial edition, Gibbs 

 connects the mathematical part of his theory of the electric cell 

 with the fundamental principles of physical chemistry, the theories of 

 van't Hoff and Arrhenius, ISTemst's osmotic theory of the Voltaic cell 



™IMd., 403. 



^Ibid., 479-81. 



118 Phil. Tr., 1887, CLXXVIL, 627, 684. 



U9 " The quantities of the different substances combined in connection with 

 the passage of a unit electricity are called the electrochemical equivalents of 

 these substances." Bryan, " Thermodynamics," 164. 



m Helmholtz, Sitzungsb. d. Berl. Akad., 1882, 22 et seq. 



