of Cellulose in Starch- grains. 251 



conversion of the cellulose, and this had led to the production 

 of the blue colour, it must have been rendered evident by dry- 

 ing this cellulose, which had been saturated with water and 

 coloured blue, and driving off the iodine by gentle heating. If 

 the cellulose had been converted into starch, this could not be 

 removed from the preparation by drying, and the blue colour 

 would necessarily make its appearance on the wetting with 

 tincture of iodine. But this does not take place ; for the cellu- 

 lose behaves exactly as before — is coloured red with chloride of 

 zinc, &c. Moreover, if cotton is treated with the ammoniated 

 oxide of copper, yet not exposed to its action long enough to pro- 

 duce solution, but only until the filaments have become swollen 

 up, the latter are coloured blue by tincture of iodine after having 

 been well washed out with water. Under this influence of the 

 ammoniacal solution of oxide of copper, the cellulose has acquired 

 the property of absorbing more water than it can in its natural 

 condition; but that we cannot imagine any conversion into 

 starch in this case, follows from the previously mentioned cir- 

 cumstance, that the cellulose does not undergo this change even 

 after complete solution, but may be precipitated with all its pro- 

 perties by salt. 



Summing up what has been stated, it becomes evident that 

 the blue coloration of cellulose by iodine by no means furnishes 

 proof that the former is wholly or partly converted into starch, 

 but that its blue colour depends solely upon the circumstance 

 that cellulose absorbs simultaneously iodine and a sufficient 

 quantity of water, while, with a smaller absorption of water, the 

 red tints are produced. 



Just as we are able to impart a pure blue colour to cellulose 

 saturated with iodine by merely adding water, we can, on the 

 other hand, by suitable operations, impart to the starch-grains 

 the property of becoming coloured red, and not blue, with 

 iodine. And in this case the red or blue colour depends, not 

 upon the quantity of iodine which we incorporate with the starch, 

 but upon the water contained in the latter. 



If we place potato-starch in water, in which it is well 

 known to become rapidly saturated, its grains begin to assume 

 colour with the addition of a minimum of iodine ; but if little 

 iodine is applied, so that only part of the grains are coloured, 

 and these very slightly, no red colour is produced, but the colour, 

 if not bright, is still decidedly blue. Very different results 

 ensue from adding an excess of iodine to the starch, allowing at 

 the same time only slight absorption of water. If, for example, 

 we place dry potato-starch in a solution of cane-sugar concen- 

 trated to the crystallizing point (which, from its great power 

 of attracting water from organic bodies, gives very little water 



