248 THE TISSUES AND MECHANISMS OF DIGESTION. 



is some multiple (n being not less than 5) of the simpler formula. A kind 

 of starch known as soluble starch, while giving a blue color with iodine, 

 forms, unlike ordinary starch, a clear solution. 



2. Dextrin*, differing from starch in forming a clear solution. Of these 

 there are at least two ; one, erythrodextrin, often spoken of simply as dextrin, 

 giving a port-wine red color with iodine, and second, achroodextrin, which 

 gives no color at all with iodine. The formula for dextrin is the same as 

 that for starch, but has a smaller molecule and might be represented by 



(c 6 H 10 o 5 r. 



3. Dextrose, also called glucose or grape-sugar, giving no coloration with 

 iodine, but characterized by the power of reducing cupric and other metallic 

 salts ; thus, when dextrose is boiled with a fluid known as Fehling's fluid, 

 which is a solution of hydrated cupric oxide in an excess of caustic alkali 

 and double tartrate of sodium and potassium, the cupric oxide is reduced 

 and a red or yellow deposit of cuprous oxide is thrown down. This reaction 

 serves with others as a convenient test for dextrose. Neither starch nor that 

 commonest form of sugar known as cane-sugar gives the reaction ; whether 

 the dextrins do is doubtful. The formula for dextrose is C 6 H, 2 O 6 ; it is more 

 simple than that of starch or dextrin and contains an additional H 2 O for 

 every C 6 . Unlike starch and dextrin it can be obtained in a crystalline 

 form, either from aqueous solutions (it being readily soluble in water), in 

 which case the crystals contain water of crystallization, or from its solutions 

 in alcohol (in which it is sparingly soluble), in which case the crystals have 

 no water of crystallization. Solutions of dextrose have a marked dextro- 

 rotatory power with rays of light. 



4. Maltose, very similar to dextrose, and like it capable of reducing 

 cupric salts. The formula is somewhat different, being C^H^On. Besides 

 this, it differs from dextrose chiefly in its smaller power i. e., a given weight 

 will not convert so much cupric oxide into cuprous oxide as will the same 

 weight of dextrose and in having a stronger rotatory action on rays of light. 

 Like dextrose it can be crystallized, the crystals from aqueous solutions con- 

 taining water of crystallization. 



Now, when a quantity of starch is boiled with water we may recognize 

 in the viscid imperfect solution, on the one hand, the presence of the starch 

 by the blue color which the addition of iodine gives rise to ; and on the 

 other hand, the absence of sugar (maltose, dextrose), by the fact that when 

 boiled with Fehling's fluid no reduction takes place and no cuprous oxide 

 is precipitated. 



If, however, the boiled starch be submitted for a while to the action of 

 saliva, especially at a somewhat high temperature such as 35 or 40 C., it 

 is found that the subsequent addition of iodine gives no blue color at 

 all or very much less color, showing that the starch has disappeared or 

 diminished ; on the other hand, the mixture readily gives a precipitate of 

 cuprous oxide when boiled with Fehling's fluid, showing that maltose or 

 dextrose is present. That is to say, the saliva has converted the starch into 

 maltose or dextrose. The presence of the previously absent sugar may also 

 be shown by fermentation arid by the other tests for sugar. Moreover, if an 

 adequately large quantity of starch be subjected to the charge, the sugar 

 formed may be isolated, and its characters determined. When this is done 

 it is found that while some dextrose is formed the greater part of the sugar 

 which appears is in the form of maltose. As is well known, starch may, by 

 the action of dilute acid, be converted into dextrin, and by further action 

 into sugar ; but the sugar thus formed is always wholly dextrose, and not 

 maltose at all. The action of saliva in this respect differs from the action 

 of dilute acid. 



