Although it is not likely that reactions involving color centers will ever be put on the precise thermodynamic basis of many reactions in liquid solutions where in many cases it has been possible to determine not only all of the species taking part in the process but their chemical activities as well, quantitative studies of the kind outlined here have proved useful in solving the problem of the composition of several absorbing centers. There remain many reactions, such as those involving theR- and N-centers and the several varieties of V-centers, particularly in crystals containing an excess of halogen, for which systematic investigations have yet to be made.