No. 502] CHEMICAL MECHANICS IX LIVING PLANT 639 



tional to the amount of actively reacting substance {or 

 substances) present. 



To carry out experiments by the procedure given above 

 is in practise very difficult and the velocities of reactions 

 are never measured by the chemist in this way. In a 

 living organism this continual bringing up of new supplies 

 of material to maintain a constant rate of change is the 

 ordinary way of life, but in the chemical laboratory pro- 

 cedure is different. There, definite amounts of sub- 

 stances are initially mixed in a vessel and the reaction is 

 allowed to progress by itself without further additions. 

 In this case there is a continual falling off of the concen- 

 tration of the substance, and so a corresponding diminu- 

 tion of the actual reaction-velocity. 



In this procedure the diminution of the initial amount 

 of substance can be actually measured by withdrawing 

 small samples at intervals of time and analyzing them. 

 Let us consider a definite example. Cane-sugar can be 

 hydrolyzed, under various conditions, to give two mole- 

 cules of hexose, according to the equation 



C 12 H 22 O n + H 2 = 2C H 12 O c . 



This reaction goes on, though extremely sowly, when an 

 aqueous solution of cane-sugar is kept very hot in a beak- 

 er. Suppose we started with, say 128 grams dissolved 

 in a liter of water and traced the diminution of this 

 amount down towards zero by withdrawing samples at 

 intervals of time and analyzing them. If we plotted the 

 sugar-content of these successive samples against the 

 times when they were taken we should get the curve given 

 in Fig. 1. If we call n minutes the time taken for the 

 sugar to diminish from 128 grams to 64 grams, we 

 should find that in the second n minutes the sugar had 

 fallen to 32 grams, after 3n minutes to 16 grams, 

 and so on, the amount halving itself every n minutes. 

 Thus the amounts of cane-sugar hydrolyzed in successive 

 equal intervals are 64, 32, 16, 8, 4, 2, 1 grams, amounts 

 in each case just exactly proportional to the quantity of 



