ii8 THE CARBOHYDRATES 



action of colloidal iron, finds that the amylo-granulose of 

 Nageli can be separated into three fractions, a, /3, and 7. 

 The a fraction is precipitated by colloidal iron alone, the /8 

 portion by colloidal iron in the presence of electrolytes, and 

 the y fraction is not precipitated by colloidal iron under any 

 Circumstances. Frcyn the action of iodine in the presence of 

 electrolytes he finds that starch contains an insoluble con- 

 stituent (amylo-cellulose) which does not react with iodine, 

 that all the soluble constituents are precipitated by iodine if 

 electrolytes are present, and that the final fraction thrown 

 down by iodine gives a brown coloration with that reagent. 

 It is therefore concluded that starch contains a number of 

 polymers varying in complexity from amylo-dextrin to amylo- 

 cellulose ; the main bulk of the grain is made up of amylo- 

 granulose a, the relative amount of dextrin and cellulose 

 being small. It was found that the reaction of iodine with 

 starch is quantitative, approximately 1600 gm. starch being 

 equivalent to 127 gm. of iodine. The opinion regarding the 

 crystalloid structure of starch is not universally held, Fischer,* 

 who believes that the grain is composed of colloid substances, 

 being a dissentient. ' 



According to Harrison, f starch grains contain no starch 

 which is really soluble in water ; the differences which obtain 

 between the inner and superficial layers are physical rather 

 than chemical, and are due to the varying conditions under 

 which they are formed. 



Action of Acids on Starch. 



The action of acids on starch varies according .to the 

 strength of the acid, the duration of the action, and the tem- 

 perature of the experiment. To complicate matters, there are 

 considerable divergences in regard to the interpretation of the 

 results obtained by the different workers. As an illustration 

 of the very different effects which may be produced under 

 different conditions, the following experiments may be 

 quoted. 



By acting on starch at the ordinary temperature with 



* Fischer : " Beih. bot. Centr.," 1905, 181. 



f Harrison : " Journ. Soc. Dyers.," 1916, 32, 40. 



