A term which may be expressed infinitesimally at time \(t\) by a ratio of differentials \(\frac{\text{d}Q_{1}(t)}{\text{d}Q_{2}(t)}\) where the kind of quantities are the same but for different components in the same system. In practice, the ratio for a finite interval is: \[\frac{\Delta Q_{1}(t_{1};\,t_{2})}{\Delta Q_{2}(t_{1};t_{2})}\] Examples are: mass change ratio, \(\frac{\text{d}m_{1}(t)}{\text{d}m_{2}(t)}\); @A00297@ change ratio, \(\frac{\text{d}n_{1}(t)}{\text{d}n_{2}(t)}\).