")S DIG 



monoxide and oxygen, when perfectly dry, cannot he made to 

 unite by means of an electric spark, but if a small quantity of 

 water vapor is present they combine readily. The following 

 equations explain the reactions. In them it is seen that water 

 takes part in the reaction, but remains finally as it was at the 

 beginning: 



2H 2 + CO + 2 = CO(OH) 2 + H,0,. 

 H 2 2 + CO = CO(OH) 2 . 

 2CO(OH) 2 = 2CO 2 + 2H 2 O. 



There is much more certainty in regard to the changes 

 produced in the bodies upon which the enzymes have acted. 

 These are in most cases hydration changes i. e., the substances 

 acted upon take up water and then break down into simpler 

 combinations. The reasons for this belief are as follows: 



1. Enzymes act only in the presence of water. 



2. In many cases an examination of the substances before 

 and after fermentation show directly a taking up of water. 



3. The action of ferments may be imitated by dilute acids 

 or alkalies, which are the most powerful hydrolytic agents 

 known. 



Salivary Digestion. The saliva is a transparent, viscid 

 fluid. It is normally alkaline in reaction. The amount formed 

 in twenty-four hours is about 1500 c.c. When taken from 

 the mouth it is known as mixed saliva, and is turbid, owing 

 to suspended particles of matter. It contains characteristic 

 salivary corpuscles, which are probably altered leukocytes. 

 Chemically it consists of 99.5 per cent, water, holding in solu- 

 tion salts, proteins, and the ferment ptyalin. The viscidity 

 is due to a glycoprotein mucin. Saliva keeps the mouth moist 

 in chewing and in speaking, dissolves certain substances, and 

 so brings them in contact with the organs of taste, makes 

 swallowing possible by wetting the food, and acts by means 

 of its ferment on starches. Ptyalin, by a process of hydration, 

 converts starch to maltose through numerous intermediate steps. 

 The first change is probably the formation of soluble starch, 

 w r hich, by further action of the enzyme, gives off a molecule 

 of maltose, leaving a body known as erythrodextrin. The latter 

 may be detected by the red color it gives upon the addition 

 of iodine. Krythrodextrin, by a further splitting off of a mal- 



