INTRODUCTION 5 



of this formula is a straight line (Fig. i). Here a 

 is the value of y, when x = o> i.e. the distance of 

 the point, in which the straight line cuts the axis 

 of ordinates, from the axis of abscissae — in Fig. i 

 #=1-5. If we put x = 1, then y = a + 6, i.e. b is the 

 distance of the point in which the straight line 

 y = a + bx cuts a vertical line x = 1 going through 

 the point 1 of the ^r-axis, from a horizontal line 



Fig. 1. 



y = a running at a distance a above the ;r-axis. 

 In Fig. 1, ^ = 0-5. 



But in most cases this very simple formula does 

 not represent the phenomenon which we wish to 

 describe. For instance, if a substance such as 

 sulphuric acid acts upon cane-sugar, this is trans- 

 formed into glucose and fructose in such a manner 

 that if we call the initial quantity of sugar 100, then 

 after a certain time / (say, one hour) only the quantity 

 80 remains unchanged ; after the time 2t (2 hours) 



