90 A TREATISE ON METAMORPHISM. 



Van't Hoff states the general law controlling chemical systems in 

 another way: "A transformation will take place of itself only in case it is 

 in a position to do a positive amount of work. If the amount of work 

 done is negative, the transformation can take place of itself only in the 

 opposite sense. If the work done is zero, it can take place in neither 

 sense."" 



EQUILIBRIUM. 



When the ions of any compound, A, unite with the ions of another 

 compound, B, so as to produce C and D, just as fast as the ions of C unite 

 with the ions of D to produce A and B the conditions are those of equi- 

 librium, or of chemical statics. When either change takes place faster 

 than the other, these are the conditions of chemical reactions, or of chemical 

 kinetics.'' When the two solutions, A and B, were first mingled, the con- 

 ditions would be those of chemical kinetics for a time — that is, until a 

 certain amount of C and D had been produced. However, when the 

 amount of C and D is sufficiently great, so that they react upon each other 

 to produce A and B as fast as A and B react to produce C and D, the 

 conditions are those of chemical statics, or equilibrium. But it is plain 

 that this does not mean that chemical activity has ceased or that there is 

 real quiescence. Interchange is taking place all the time; but as this inter- 

 change is compensatory, no heat effect is produced, and the total quantity 

 of each of the compounds present remains the same. The equilibrium is 

 therefore really dynamic. 



HOMOGENEOUS AND HETEROGENEOUS SYSTEMS. 



A chemical system is homogeneous " when it has the same physical and 

 chemical nature at every point." When this is not the case it is heteroge- 

 neous." A solution is therefore homogeneous. If a solid substance be also 

 present, the system is heterogeneous. A heterogeneous system consists "in 

 the intimate association of different complexes, each of which is homoge- 

 neous in itself, such as solid salts and saturated solutions.'" 2 Each of these 

 complexes is called a phase of the system. "The condition of equilibrium 

 of a heterogeneous system is independent of the relative quantity by weight 



"Jones, H. C, On the increasing importance of inorganic chemistry; Science, new series, vol. 8, 

 1898, p. 930. 



&Nernst, W., Theoretical chemistry, translated hy C. S. Palmer, Macmillan & Co., London, 1895, 

 pp. 358-360. 



cNernst, cit, p. 357. 



tfNernst, cit., p. 391. 



