ALIMENTATION AND DIGESTION 105 



clear solution, like salt or sugar, but an opalescent liquid, 

 which does not become clear by passing through ordinary 

 filter paper. A characteristic test for starch the blue color 

 produced when a few drops of a solution of iodine * are 

 added to it may be used to detect its presence in the 

 following experiments : 



EXPERIMENT I 



Two test tubes or small beakers containing starch paste are prepared. 

 Collect some saliva and boil half of it. To one portion of the starch 

 paste add the boiled saliva (after it has again cooled to the room tem- 

 perature) ; to the other add the unboiled saliva. Mere observation will 

 show that while the first test tube remains opalescent, the second soon 

 becomes clear. A few minutes after this change has occurred, a little 

 of the second starch-saliva mixture may be removed, diluted with water, 

 and tested with iodine ; the color produced is no longer pure blue, but 

 purplish ; that is, a mixture of red and blue. Some minutes later the 

 iodine test gives a port-wine red color, and stiil later no color at all. 

 This change of reaction is due to the fact that the saliva has changed 

 the starch into dextrine, which gives the red color, and then has changed 

 the dextrine into a substance which gives no color with iodine. 2 Mean- 

 while the starch in the first test tube shows no change either in its 

 opalescent appearance or in its original blue reaction with iodine. 



Boiling the saliva has destroyed its power of acting on 

 starch, and it is known that this is due to the fact that 

 the heat has destroyed the enzyme, known as ptyalin, or 

 salivary diastase, which has the power of changing starch 

 to sugar. 



1 Made by dissolving a few flakes of iodine in alcohol or in an aqueous 

 solution of potassium iodide. 



2 The cleavage of the starch molecule does not take place by splitting off 

 successive molecules of dextrose, but by splitting into two molecules, each, 

 let us say, approximately half as large as the original molecule. By some 

 such process first one, then another, dextrine successively appears. Con- 

 tinuation of the cleavage ultimately gives a substance, maltose, which 

 consists of two molecules of dextrose bound together. Finally, the maltose 

 is split into two molecules of grape sugar. We speak of the dextrines and 

 maltose as intermediate products, and of the dextrose as the end product, of 

 the cleavage. 



