252 H. von Mohl on the supposed Existence 



to the starch-grains), and add tincture of iodine, the starch- 

 grains do not become blue, but are coloured various tints, from 

 bright wine-red to violet. The colours change to blue directly 

 water is applied. The same result is obtained when dried potato- 

 starch is placed in absolute alcohol, a considerable quantity of 

 iodine dissolved in this, and water dropped in carefully and with 

 frequent agitation, until the starch-grains are coloured: their 

 colour will be from reddish to deep violet, but none will appear 

 blue. A greater addition of water at once brings out the blue 

 colour. 



Analogous phenomena are displayed during the action of 

 chloro-iodide of zinc upon dry starch. The latter is capable of 

 abstracting water from the solution, and gradually becomes 

 strongly swollen up through the influence of chloride of zinc. 

 But this process requires much time ; and the outermost layer 

 of the granules especially opposes at first a resistance to the 

 expansion. Hence the grains appear at first brown-red, like 

 dry starch-grains saturated with iodine ; subsequently this colour 

 changes while they swell up (which often occurs irregularly and 

 partially), into a bright blue. If, however, the quantity of the 

 starch in proportion to the chloride of zinc solution is great, 

 and, after complete swelling-up, forms with it a very tough, 

 dense, and glutinous mass, the bright blue colour changes in 

 the course of twenty-four or thirty-six hours, during the swell- 

 ing-up and while the toughness of the mass increases, into a 

 fine purple- red. If water is now added without fresh iodine, 

 the colour changes rapidly into blue. Here evidently, at the 

 commencement of the swelling-up of the grains, the quantity of 

 water in the zinc solution is sufficient to supply the half-swollen 

 granules with enough water for the production of the blue 

 colour, while with the increasing expansion of the starch-grains 

 it becomes again insufficient, and thus the blue colour is brought 

 back to red. 



If I have rightly comprehended the above-described pheno- 

 mena, the reaction of cellulose and starch with iodine, far from 

 being the only test by which to distinguish them, affords, on the 

 contrary, a character of no value for distinguishing between the 

 two chemical compounds. 



Strictly speaking, Nageli did not regard the colour which the 

 said substances take with iodine as the only distinction between 

 them ; for he mentions as a second the circumstance that cellu- 

 lose withstands solvents more strongly than starch, while starch- 

 grains, on account of the cellulose they contain, are caused to 

 swell up and dissolve with more difficulty than pure starch would 

 be (p. 193). This statement is certainly true in respect to most 

 solvents, and the complete insolubility of cellulose in water must 



