VIII.] SALIVARY DIGESTION. 69 



This is best done through a filter perforated at its apex by a pin- 

 hole. In this way all air-bubbles are got rid of. Label three 

 test-tubes A, B, and C. In A place starch mucilage, in B saliva, 

 and in C I volume of saliva and 3 volumes of starch mucilage. 

 Place them in a water-bath at 40° C. for ten minutes. Test for a 

 reducing sugar in portions of all three, by means of Fehling's 

 solution. A and B give no evidence of sugar, while C reduces 

 the Fehling, giving a yellow or red deposit of cuprous oxide. 

 Therefore, starch is converted into a reducing sugar by the saliva. 

 This is done by the ferment ptyalin contained in it. 



{b.) Test a portion of C with solution of iodine ; no blue colour 

 is obtained, as all the starch has disappeared, being converted 

 into a reducing sugar or maltose. 



(r.) Make a thick starch mucilage, place some in test-tubes 

 labelled A and B. Keep A for comparison, and to B add saliva, 

 and expose both to 40° C. A is unaffected, while B soon becomes 

 fluid — within two minutes — and loses its opalescence ; this liquefac- 

 tion is a process quite antecedent to the saccharifying process 

 which follows. 



4. Stages between Starch and Maltose. — Mix starch and saliva 

 as in 3 (a.) C, and place in a water-bath at 40° C. At intervals of a 

 minute test small portions with iodine. Do this by taking out a 

 drop of the liquid by means of a glass rod. Place the drop on a 

 white porcelain plate, and with another glass rod add a drop of 

 iodine solution. 



Note the following stages : — At first there is pure blue with 

 iodine due to the soluble starch formed giving also a blue with 

 iodine, later a deep violet, showing the presence of erythro-dextrin, 

 the violet resulting from a mixture of the red produced by the 

 dextrin and the blue of the starch. Then the blue reaction entirely 

 disappears, and a reddish-brown colour, due to erythro-dextrin 

 alone, is obtained. After this the reaction becomes yellowish- 

 brown, and finally there is no reaction with iodine at all, achroo- 

 dextrin being formed, along with a reducing sugar or maltose. 

 The latter goes on forming after iodine has ceased to react with the 

 fluid, and its presence is easily ascertained by Fehling's solution. 

 The soluble starch is precipitated by alcohol, while maltose is 

 not. In this way this body may be separated. 



5. Effect of Different Conditions on Salivary Digestion. 



{a. ) Label three test-tubes A, B, aVid C. Into A place some saliva, boil it, 

 and add some starch mucilage. In B and C place starch mucilage and saliva, 

 to B add a few drops of hydrochloric acid, and to C caustic potash. Place 

 all three in a water-bath at 40" C. , and after a time test them for sugar by 

 Fehling's solution. No sugar is formed — in A because the ferment was de- 



