Wells & Foote — One Hundred Years of Chemistry. 293 



analytical chemistry in a book upon this subject which he 

 published. 



The Phase Rule. — In 1876 Willard Gibbs of Yale pub- 

 lished a paper in the Proceedings of the Connecticut 

 Academy of Science on the "Equilibrium of Heteroge- 

 neous Substances," and two years later he published an 

 abstract of the article in the Journal (16, 441, 1878). He 

 had discovered a new law of nature of momentous 

 importance and wide application which is called the 

 " Phase-Rule " and is expressed by a very simple 

 formula. 



The application of this great discovery to chemical 

 theory was delayed for ten years, partly, perhaps, 

 because it was not sufficiently brought to the attention of 

 chemists, but largely it appears because it was not at 

 first understood, since its presentation was entirely 

 mathematical. 



It was Rooseboom, a Dutch chemist, who first applied 

 the phase-rule. It soon attracted profound attention, 

 and the name of Willard Gibbs attained world-wide fame 

 among chemists. When Nernst, who is perhaps the most 

 eminent physical chemist of the present time, was deliv- 

 ering the Silliman Memorial Lectures at Yale a few years 

 ago, he took occasion to place a wreath on the grave of 

 Willard Gibbs in recognition of his achievements. 



To understand the rule, it is necessary to define the 

 three terms, introduced by Gibbs, phase, degrees of free- 

 dom and component. 



By the first term, is meant the parts of any system of 

 substances which are mechanically separable. For 

 instance, water in contact with its vapor has two phases, 

 while a solution of salt and water is composed of but one. 

 The degrees of freedom are the number of physical con- 

 ditions, including pressure, temperature and concentra- 

 tion, which can be varied independently in a system 

 without destroying a phase. The exact definition of a 

 component is not so simple, but in general, the com- 

 ponents of a system are the integral parts of which it is 

 composed. Any system made up of the compound HoO, 

 for instance, whether as ice, water or vapor, contains but 

 one component, while a solution of salt and water con- 

 tains two. Letting P, F. and C stand for the three terms, 

 the phase-rule is simply 



F=C+2-P 



