of Cellulose in Starch-grains. 249 



bine and become coloured in the same way with iodine. The 

 coloration stands altogether on the boundaries between physical 

 and chemical phsenomena, and must indeed be connected in the 

 first place with the capacity possessed by various cell-membranes 

 which swell up rather strongly in water, to attract the colouring 

 matter from a solution of carmine in ammonia. 



As long as no other means was known by which cellulose 

 could be coloured blue with iodine, except the simultaneous 

 action of sulphuric acid, the conjecture was certainly not far- 

 fetched that this acid transformed the cellulose into starch. 

 Hence this view was many times put forth, for instance by 

 Mitscherlich : I have always regarded it as erroneous, because 

 the cellulose swollen up by means of sulphuric acid has none of 

 the other characters of starch. This assumption is now posi- 

 tively refuted by the researches of Bechamp, who has shown 

 that the subsequent products of the conversion of cellulose by 

 sulphuric acid are essentially different from the corresponding 

 transformation of products of starch, which could not be the 

 case if sulphuric acid converted cellulose in the first place into 

 starch. 



Long before this was known, the belief in the conversion of 

 cellulose into starch by sulphuric acid, founded on the blue 

 coloration with iodine, must have been shaken by my demon- 

 strating that the action of sulphuric acid was by no means ne- 

 cessary for the blue coloration of cellulose, but that it sufficed 

 to saturate the cellulose with iodine (which in its usual condi- 

 tion it does not readily absorb) before applying water to it ; 

 then, if the cellulose is made to absorb water, the iodine renders 

 it blue, until itself is completely extracted again by the water 

 surrounding the preparation. Nageli endeavours to explain this 

 process also by a chemical conversion of the cellulose; for he 

 conjectures that the iodine gives origin to iodic and hydriodic 

 acids, which transform the cellulose. This is an entirely arbitrary 

 and groundless hypothesis. It should at least have been de- 

 monstrated by experiment that these acids have the power of 

 acting in the same manner upon cellulose as sulphuric acid, and 

 of producing a blue colour in it in the presence of iodine. I 

 have made the experiment of saturating purified cellulose with 

 tincture of iodine, and adding the said acids ; they caused neither 

 a visible action upon the cellulose nor a trace of blue coloration. 



But, without this demonstration that the said acids do not 

 bring about a conversion of the cellulose and its blue coloration, 

 it may be recognized from the behaviour of the cellulose satu- 

 rated with iodine, that it is not transformed into starch. Iodine 

 affects starch infinitely more readily than it does cellulose, so 

 that starch absorbs the iodine out of water when only a minimum 



