HETEROGENEOUS SYSTEMS. 91 



in which each phase is present in the system."" To illustrate, if an excess 

 of salt be in a solution, so that it is saturated, and an additional amount 

 of salt be added, this does not in the least change the quantity of salt 

 held in a given volume of the solution. Therefore the equilibrium in a 

 saturated solution is independent of the amount of undissolved salt in the 

 solution. It follows that in a heterogeneous system "the condition of 

 equilibrium is independent of the relative mass of each of the phases." 6 A 

 simple case of heterogeneous equilibrium is that between ice and liquid 

 water, or between liquid water and water vapor. " For a definite external 

 pressure there corresponds a definite temperature at which the two systems 

 can exist beside each other ; thus ice and water are coexistent at atmos- 

 pheric pressure at 0° C; and liquid water and water vapor, at atmospheric 

 pressure and at 100° C. If we change the external pressure, at a tempera- 

 ture which is kept constant, or if we change the temperature, at an external 

 pressure which is kept constant, then the reaction advances" 1 to equilibrium 

 in one direction or the other. " The process is ended as soon as the 

 expansive force of the evaporating or dissolving substance is held in equi- 

 librium by the gas pressure of the vaporized molecules or by the osmotic 

 pressure of the dissolved molecules, respectively." 



NATURE AND SPEED OF REACTIONS. 



The fundamental principle of chemical dynamics is that chemical action 

 is proportional to the active mass. d This is the law of mass action. 



The speed of a chemical reaction which occurs under any given con- 

 ditions depends upon the compounds, the strength of the solutions, the 

 mechanical action, and the heat. Hence each of these features requires con- 

 sideration. 



THE COMPOl'XDS. 



The reactions depend upon the compounds present, or, in other words, 

 upon the nature of the ions composing them ; for the conditions under which 

 two ions, A and B, unite may be different from those under which one of 

 these ions will unite with a third, as A with C, or different from those under 

 which two other ions, C and D, unite. In order that ions shall unite in 

 solution they must meet or come within the limits of molecular attraction of 



«Nernst, cit., pp. 391-392. 

 »Nernst, cit, p. 393. 

 cNernst, cit., p. 403. 

 ' *Ostwald, W., Outlines of general chemistry, translated by James Walker, Macmillan & Co., 2d ed., 

 London, 1895, p. 292. 



