DIGESTION AND ABSORPTION OF NUTRIENTS 95 



130. Digestion of mineral matters. (Optional.) Laboratory 

 demonstration. 



Note to Teacher: Part A should be demonstrated in connection 

 with the study of saliva ; Part B ; in connection with gastric diges- 

 tion. 



Materials: Table salt, phosphate of lime, diluted hydrochloric 

 acid (one part acid to six parts water). 



A. Soluble mineral matters. 



1. Put some table salt into a test tube, add water, and shake 



well. Does the salt dissolve ? How do you know ? 



2. Saliva is largely (over 99 per cent) composed of water. 



How, then, are soluble mineral matters made liquid in 

 the mouth ? 



B. Insoluble mineral matters. 



1. Put some insoluble mineral matter like phosphate of lime 



(which is one of the constituents of milk) into a test 

 tube, add water, and shake well, then allow the tube 

 to stand for a time before answering the following 

 questions. 



a. Does phosphate of lime dissolve in water ? How do you 



know ? Why is phosphate of lime called an insoluble 

 mineral matter ? 



b. Shake the mixture again and add some diluted hydro- 



chloric acid. What change do you observe ? 



2. Hydrochloric acid is one of the ingredients of gastric juice. 



How, then, are insoluble mineral matters like phos- 

 phate of lime digested in the stomach ? 



131. Digestion of proteins. One of the most important 

 actions which takes place in the stomach is the digestion of 

 proteins. This class of nutrients is not readily soluble in 

 water and so cannot pass through the walls of cells (P. B., 52). 

 Hence, before proteins can be made available for use in the 

 body they must be changed to a soluble form known as 



