Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 315 



since the molecule or 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^H^O^. 



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

 coloration with iodine, but characterised 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, give this reaction; whether the dextrins do is doubtful. 

 The formula for dextrose is C, ; H l2 6 ; it is more simple than that 

 of starch or dextrin and contains an additional H 2 for every C, ; . 

 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^rL.O,,. 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 

 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 colour at all, or very much less colour, shewing that 

 the starch has disappeared or diminished ; on the other hand the 

 mixture readily gives a precipitate of cuprous oxide when boiled 



