CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 359 



must bear in mind the existence of the following bodies all 

 belonging to the class of carbohydrates. 



1. Starch) which forms with water not a true solution but a 

 more or less viscid mixture, and gives a characteristic blue colour 

 with iodine. The formula is C 6 H 10 6 or more correctly (C 6 H 10 5 ) n 

 since the molecule of starch 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 colour with iodine, forms, unlike 

 ordinary starch, a clear solution. 



2. Dextrins, 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 colour with iodine, and 

 a second, achroodextrin, which gives no colour 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 6 ) n ,. 



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

 colouration with iodine, but characterised by the power of re- 

 ducing 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, give this reaction; whether the dextrins do is doubtful. 

 The formula for dextrose is C 6 H 12 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 crystallisation, or from 

 its solutions in alcohol (in which it is sparingly soluble), in which 

 case the crystals have no such water of crystallisation. Solutions 

 of dextrose have a marked dextrorotatory power over rays of light. 



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

 reducing cupric salts. The formula is somewhat different, being 

 C 12 H 22 U . Besides this, it differs from dextrose chiefly in its 

 smaller reducing 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 crystallised, the crystals from aqueous- 

 solutions containing water of crystallisation. 



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

 recognise in the viscid imperfect solution, on the one hand the 

 presence of starch, by the blue colour 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 



