FOODS AND THE THEORY OF DIGESTION 131 



How Substances are Liquefied. While the reduction of 

 solids to the liquid state is accomplished in some instances 

 by heating them until they melt, they are more frequently 

 reduced to this state by subjecting them to the action of 

 certain liquids, called solvents. Through the action of the 

 solvent the minute particles of the solid separate from 

 each other and disappear from view. (Shown in dropping 

 salt in water.) At the same time they mix with the solvent, 

 forming a solution, from which they separate -only with 

 great difficulty. For this reason solids in solution can dif- 

 fuse through porous partitions along with the solvents in 

 which they are dissolved (page 73). 



By digestion the nutrients are reduced to the form of a 

 solution. The process is, simply speaking, one of dissolv- 

 ing. The liquid employed as the digestive solvent is water. 

 The different nutrients dissolve in water, mixing with it 

 to form a solution which is then passed into the body 

 proper. 



Digestion not a Simple Process. Digestion is by no 

 means a simple process, such, for instance, as the dissolv- 

 ing of salt or sugar in water. These, being soluble in 

 water, dissolve at once on being mixed with a sufficient 

 amount of this liquid. The majority of the nutrients, how- 

 ever, are insoluble in water and are unaffected by it when 

 acting alone. Fats, starch, and most of the proteids do 

 not dissolve in water. Before these can be dissolved they 

 have to be changed chemically and converted into sub- 

 stances that are soluble in water. This complicates the 

 process and prevents the use of water alone as the digestive 

 solvent. 



A Similar Case. If a piece of limestone be placed in 

 water, it does not dissolve, because it is insoluble in water. 

 If hydrochloric acid is now added to the water, the lime- 



