Josiah Willard Gibbs. 291 



rule, giving a limit to the number of different substances actually 

 present in terms of the number of phases that can co-exist. In fact, the 

 great service of Gibbs to general chemistry was his definite marking 

 out of the channels within which a scheme of reactions can proceed, by 

 aid of a discussion of the relations of co-existing states or phases of the 

 material, of so general and abstract a character that it could only be 

 carried through effectively by his graphical regional representations, 

 which have shown their power in the application of the method to 

 special problems ever since. 



In subsequent parts of the memoir the phenomena of interfacial 

 films were considered in relation to the physical and chemical con- 

 stitution of the films, leading for the first time to an insight into the 

 causes of their permanence, which has since been expanded, mainly by 

 Lord Eayleigh. Here his essential problem was to try to formulate 

 conditions for interaction, such as would initiate new substances at the 

 interface where two systems are in contact. Not to recur again to 

 the complete establishment of osmotic principles almost before the 

 phenomena had attracted quantitative attention, and to delicate 

 mathematical applications to the effect of stress on chemical equi- 

 librium of crystalline solids in their mother liquid, which make precise 

 James Thomson's early ideas but have hardly yet borne fruit, the 

 memoir ends, in 1878, by the fundamental application to voltaic 

 phenomena. The electromotive force in a reversible cell turns out 

 to be the available energy, not the total energy, liberated per electro- 

 chemical equivalent of decomposition, and it is shown by actual cases 

 that this distinction may play an essential part. One brilliant 

 illustration may be quoted ; it cannot matter to the available energy 

 at the temperature of liquefaction whether a substance is solid or 

 fused, for the transfer of the latent heat at the uniform temperature 

 can produce no mechanical effect ; accordingly electromotive forces 

 should not alter abruptly owing to solidification or fusion, thus 

 explaining a striking fact that had already been experimentally noted. 

 This electrical application of the theory had been developed indepen- 

 dently, by a less general analysis but with extensive experimental 

 confirmations, by Helmholtz in 1882, in ignorance of Gibbs' work. It 

 was not till an even later time that the essential bearings of the theory 

 on general chemistry were thrown into practical light and developed 

 experimentally by van der Waals, van't Hoff, and the other great 

 physical chemists of the Dutch school. In England the work was 

 earlier known. Towards the end of his career (1876) Maxwell 

 contributed an enthusiastic exposition to the Cambridge Philosophical 

 Society, confined to the first part of the memoir, the only part then 

 published, and conveying the opinion that by utilising fully the 

 fundamental concepts of energy and entropy, the author seemed to 



