12 INTRODUCTION 



is indicated by putting y l = 2y and T = a + 2x (see Fig. 

 9, coagulation of haemoglobin). In the next figure 

 other reductions of the scale have been introduced 

 which are easily seen from the indications. 



In some cases the differences between observed 

 and calculated values are so small that they cannot 

 be represented in diagrams if these are not given on 

 a very large scale. Under such circumstances it is 

 preferable to give the values observed side by side 

 with those calculated. 



Before we consider the laws governing the re- 

 actions of the substances treated below, which have 

 not been prepared in a pure state, it will be worth 

 while to recall their general properties in order 

 that we may be familiar with them and understand 

 why so much work has been done on their examina- 

 tion. It is quite clear that if these substances did 

 not exert some very obvious and important actions, 

 they would probably have escaped our observation. 

 In reality these substances, products of animal or 

 vegetal bodies, are found to govern the chemical 

 processes going on in living bodies. The most 

 important for animal life are the juices secreted by 

 the digestive tract. To begin with, the salivary 

 glands give a juice containing ptyalin, which trans- 

 forms the insoluble starch of the food into the 

 soluble sugar maltose. Then glands in the walls of 

 the stomach secrete the stomachical juice, which con- 

 tains two active substances, \he pepsin, which decom- 

 poses the albuminous substances of the food into 



