334 HISTORY OF CHEMISTRY. [LECTURE XVI. 



representing our science and be looked upon as of equal 

 value with them. 



In now passing on to the subject itself, I begin, in the first 

 place, with the law of mass action, already mentioned on p. 315, 

 which is apparently destined to play a constantly increasing 

 part. 



These chemical studies received a new direction and a 

 fresh stimulus from the theory of phases, due to Gibbs. 1 The 

 phase rule developed by him, and proved both by him and 

 afterwards by van der Waals, 2 is to the following effect : 

 Complete equilibrium can only exist when the number of 

 phases present exceeds the number of components by one. 

 By phases, are understood homogeneous portions of a system. 

 Each state of aggregation represents at least one phase. In the 

 solid or the liquid state, two or more different phases may exist ; 

 a gas, however complex, can only form one phase. Complete 

 equilibrium is a condition which depends only on the tempera- 

 ture, and is mostly definable by a certain value of the pressure. 

 By components are understood all those chemical elements 

 taking part in the equilibrium, whose quantities are subject to 

 independent variation. 3 Ammonium chloride, for example, 

 has only one component, it being a matter of indifference 

 whether we choose nitrogen, hydrogen, or chlorine. If excess 

 of ammonia or of hydrochloric acid is added, there are then 

 two independent components. Calcium carbonate, above its 

 dissociation temperature, has two independent components, 

 calcium and carbon ; for the composition of the solid phases 

 calcium carbonate and calcium oxide cannot be determined 

 by the amount of calcium alone. Hence complete hetero- 

 geneous equilibrium is established in the case of ammonium 

 chloride with two phases, and in the case of calcium carbonate 

 with three phases. 



1 Trans. Connecticut Acad. 3, 108 and 343 (1876) ; German Translation 

 by W. Ostwald, Leipzig 1892. 2 Rec. Trav. Chim. 6, 265, communicated 

 by Roozeboom. a I follow here the exposition by Planck (see Article 

 Thermochemie, in Ladenburg's Handworterbuch der Chemie. II, 636). 



