For the case where the rate of exchange is measured by the growth of activity, y, in B y{t) = y (oo) [1 -exp (-*)](4) d In s/d In (y/y(°>)) = (exp(s) -l)/s = F(s) (5) The figure contains a plot of this function, too. In this case, of course, the fractional error in y cannot possibly be constant as y approaches zero; an estimate of -y as a function of y is needed for selecting an optimum reaction time or calculating the error of a particular determination.Contribution No. 1233 from the Gates and