28 THE STORY OF THE LIVING MACHINE. 



eligible 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, in- 

 asmuch as they are able to bring about the chem- 

 ical 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 chem- 

 ical affinity. Chemists have found simple methods 

 of producing this chemical union, and the manu- 

 facture of sugar out of starchy material has even 

 become something of a commercial industry. One 

 of the methods by which this change can be pro- 

 duced 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 mole- 

 cule and forms sugar. Now we do not understand 

 how this saliva possesses this power to induce the 

 chemical change. But apparently the process is 

 of the simplest character and involves no greater 

 mystery than chemical affinity. We know that the 

 saliva contains a certain material called a ferment, 

 which is the active agent in bringing about the 

 change. This ferment is not alive, nor does it 

 need any living environment for its action. It 

 can be separated from the saliva in the form of a 

 dry amorphous powder, and in this form can be 



