IS THE BODY A MACHINE ? 39 



since they will take place equally well in the 

 chemist's laboratory. They are simply changes 

 in the molecular structure of the food material, 

 and only such changes as are simple and familiar 

 to the chemist. The forces which effect the 

 change are undoubtedly those of chemical affinity. 

 The only feature of nhe process which is not 

 perfectly intelligible in terms of chemical law is 

 the nature of the digestive juices. The digestive 

 fluids of the mouth and stomach contain certain 

 substances which possess a somewhat remarkable 

 power, inasmuch as they are able to bring about 

 the chemical changes which occur in the digestion 

 of food. An example will make this clearer. 

 One of the digestive processes is the conversion 

 of starch into sugar. The relation of these two 

 bodies is a very simple one, starch being readily 

 converted into sugar by the addition to its 

 molecule of a molecule of water. The change can^ 

 not be produced by simply adding starch to water, 

 but the water must be introduced into the starch 

 molecule. This change can be brought about in 

 a variety of ways, and is undoubtedly effected by 

 the forces of chemical affinity. Chemists have 

 found simple methods of producing this chemical 

 union, and the manufacture of sugar out of 

 starchy material has even become something of a 

 commercial industry. One of the methods by 

 which this change can be produced is by adding to 

 the starch, along with some water a little saliva. 

 The saliva has the power of causing the chemical 

 change to occur at once, and the molecule of water 

 enters into the starch molecule and forms sugar. 

 Now we do not understand how this saliva pos- 



