PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 509 
solution, the active mass of each reacting molecular species (which 
are, according to the current views, ions and undissociated mole- 
cules) is given by the concentration of that particular species in the 
unit of volume (expressed always in molecular, or equivalent, con- 
centrations). It should be observed that K is found to be constant 
only in dilute solutions (and correspondingly, in gaseous systems 
only at low pressures); but there is no doubt that its lack of con- 
stancy at greater concentrations (or pressures) is due altogether to 
our lack of accurate knowledge of the real concentrations (partial 
pressures) of the reacting molecular species. The reaction con- 
stant K is independent of the amounts of the various substances 
originally present, so long as the temperature and pressure remain 
constant; its dependence on temperature (at constant pressure) is 
given by the equation’ 
dinK Q 
ae (111) 
Hence if we know K at any one temperature, and the heat change 
Q accompanying the reaction,” we can calculate A at any tempera- 
ture; conversely from the values of A at various temperatures, we 
can compute mean values of Q. 
Similarly, the variation of K with (uniform) pressure, at con- 
stant temperature, is given by the equation 
dinK AV 
i ee 
(IV) 
where AV is the increase of volume accompanying the reaction 
when 1 mol. is transformed. This equation holds strictly in homo- 
geneous systems; where solid substances are present, it is essential 
that they be subject to the same uniform pressure as the rest of the 
system. The equation is the quantitative formulation of the state- 
ment that increase of pressure tends to displace the equilibrium in 
the direction in which the reaction is accompanied by decrease of 
volume. 
tIn K is log, K=2.303 log,, K. 
2 For exact work we must know further how Q varies with temperature; the 
assumption of a constant value of Q, however, leads to results sufficiently exact for 
most purposes. Further, our knowledge of Q at any one temperature usually leaves 
much to be desired, not to speak of its variation with temperature. 
