56 INTRODUCTION. 



ditions necessary for their accomplishment exist in the body, and 

 do not exist elsewhere. All chemical phenomena are liable to be 

 modified by surrounding conditions. Many reactions, for example, 

 which will take place at a high temperature, will not take place at 

 a low temperature, and vice versa. Some will take place in the light, 

 but not in the dark ; others will take place in the dark, but not in 

 the light. If a hot concentrated solution of sulphate of soda be 

 allowed to cool in contact with the atmosphere, it crystallizes; 

 covered with a film of oil, it remains fluid. Because a chemical 

 reaction, therefore, takes place under one set of conditions, we can- 

 not be at all sure that it will also take place under others, which 

 are different. 



The chemical conditions of the living body are exceedingly com- 

 plicated. In the animal solids and fluids there are many substances 

 mingled together in varying quantities, which modify or interfere 

 with each other's reactions. New substances are constantly entering 

 by absorption, and old ones leaving by exhalation ; while the circu- 

 lating fluids are constantly passing from one part of the body to 

 another, and coming in contact with different organs of different 

 texture and composition. All these conditions are peculiar, and so 

 modify the chemical actions taking place in the body, that they are 

 unlike those met with anywhere else. 



If starch and iodine be mingled together in a watery solution, 

 they unite with each other, and strike a deep opaque blue color ; 

 but if they be mingled in the blood, no such reaction takes place, 

 because it is prevented by the presence of certain organic substances 

 which interfere with it. 



If dead animal matter be exposed to warmth, air, and moisture, 

 it putrefies ; but if introduced into the living stomach, even after 

 putrefaction has commenced, this process is arrested, because the 

 fluids of the stomach cause the animal substance to undergo a 

 peculiar transformation (digestion), after which the bloodvessels 

 immediately remove it by absorption. There are also certain sub- 

 stances which make their appearance in the living body, both of 

 animals and vegetables, and which cannot be formed elsewhere ; 

 such as fibrin, albumen, casein, pneumic acid, the biliary salts, mor- 

 phine, &c. These substances cannot be manufactured artificially, 

 simply because the necessary conditions cannot be imitated. They 

 require for their production the presence of a living organism. 



The chemical phenomena of the living body are, therefore, not 

 different in their nature from any other chemical phenomena ; but 



