7O PRACTICAL PHYSIOLOGY. [VIII. 



troyed by boiling, and in B and C because strong acids and alkalies arrest 

 the action of ptyalin on starch. 



(ft.) If a test-tube containing starch mucilage and saliva be prepared as in 

 3 (a.) C, and placed in a freezing mixture, the conversion of starch into a re- 

 ducing sugar is arrested ; but the ferment is not destroyed, for on placing the 

 test-tube in a water-bath at 40 C., the conversion is rapidly effected. 



(c.) Mix raw starch with saliva and keep it at 40 C. Test it after half an 

 hour, when little or no sugar will be found. 



6. Starch is a Colloid, but Sugar is a Crystalloid and dialyses. 



(a.) Place in a sausage parchment tube (p. 78), 20 cc. of starch mucilage (A), 

 and into another, some starch mucilage with saliva (B). Suspend A and B 

 in distilled water in separate vessels. 



(.6.) After some hours test the diffusate in the distilled water. No starch 

 will be found in the diffusate of either A or B, but sugar will be found in that 

 of B, proving that sugar dialyses, while starch does not. Hence the necessity 

 of starch being converted into a readily diffusible body during digestion. 



7. Action of Malt-Extract on Starch. 



(a.) Rub up 10 grams of starch with 30 cc. of distilled water in a mortar, 

 add 200 cc. of boiling water, and make a strong starch mucilage. 



(ft.) Powder 5 grams of pale low-dried malt, and extract it at 50 C. for half 

 an hour with 30 cc. of distilled water, and filter. Keep the filtrate. 



(c.) Place the starch paste of (a.) in a flask, and cool to 60 C., add the ex- 

 tract of (ft.), and place the Hask in a water-bath at 60 C. 



(d. ) Observe that the paste soon becomes fluid, owing to the formation of 

 soluble starch, and if it be tested from time to time (as directed in 4), it gives 

 successively the tests for starch and erythro-dextrin. Continue to digest it 

 until no colour is obtained with iodine i.e., until all starch and erythro- 

 dextrin have disappeared. 



(<?.) Take a portion of (d.) and precipitate it with alcohol = achroo-dextrin. 

 The liquid also contains maltose (/'.). 



(/.) Boil the remainder of the fluid, cool, filter, and evaporate the filtrate to 

 20 cc. Add 6 volumes of 90 per cent, spirit to precipitate the dextrin ; boil, 

 filter, and concentrate to dryness on a water-bath and dissolve the residue in 

 distilled water. The solution is maltose (C^H^O^ +H 2 0) If the alcoholic 

 solution be exposed to air, crystals of maltose are formed. 



ADDITIONAL EXERCISES. 



8. Compare the Reducing Power of Maltose and Dextrose. 



(a.) With Fehling's solution estimate the reducing power of the solution 

 obtained in 7 (/. ). (See " Urine. ") 



(ft.) Boil in a tlask for half an hour 50 cc. of the solution of maltose with 

 5 cc. of hydrochloric acid. Neutralise with caustic soda, and make up the 

 volume, which has been reduced by the boiling, to 50 cc., and determine by 

 Fehling's solution the reducing power. The acid has converted the maltose 

 into dextrose, and the ratio of the former estimation (.) to the present one 

 should be 65 to 100. 



(c.) A solution of pure dextrose treated as in (ft.) is not affected in its re- 

 ducing power. 



Saliva has practically the same effect on starch as malt-extract, and may be 

 used instead of the latter. 



