10 INTRODUCTION 



acquainted with them and therefore lose sight of 

 the relation connecting z with u t which is, however, 

 presented to the eye by the curve representing 

 z = ^(u). 



In some cases the function f(z) or p(u) within a 

 certain interval coincides very nearly with a function 

 which is familiar to us. Thus, for instance, when 

 we investigate the influence of temperature upon 

 the velocity of a reaction, we find that the velocity 

 of reaction, K, increases nearly in a geometrical pro- 

 gression, when the temperature, t, increases in an 

 arithmetical one. For small intervals of temperature 

 this rule is very nearly exactly true. Then we make 

 use of this circumstance, and as in Fig. 2 we plot 

 log K as a function of t. When, as below in Fig. 

 9, the observations fall within an interval of tem- 

 perature less than io° C, the deviation of the strict 

 formula from a linear equation is so small, that it 

 falls wholly within the errors of observation, and we 

 make use of the rectilinear representation. But if 

 the said interval exceeds io° C. the divergence 

 between the strict formula and a linear equation is , 

 so great that we cannot use the straight line as a 

 true expression of the observed data, but make use 

 of the representation of the strict formula. But even 

 in this case we use log K, and not K itself, for the 

 representation, because the curve then has a nearly 

 rectilinear form, that is, its curvature is very in- 

 significant, and the smaller the curvature is, the 

 clearer is the representation given by the curve 

 to the eye, and correspondingly, the easier it is 



