250 H. von Mohl on the supposed Existence 



of iodine is contained in the latter. At the same time the 

 iodine adheres very firmly to starch, so that repeated washings 

 with much water are required to extract the iodine from blued 

 starch. It is quite different with cellulose saturated with iodine : 

 it certainly also very quickly becomes blue when water is added, 

 for it already contains the iodine ; but it retains the latter with 

 very little power, and quickly parts with it to the surrounding 

 water. The cellulose bleached in this way is just as difficult to 

 colour blue with iodine a second time, and takes up as little 

 from an aqueous solution as cellulose which has never been in 

 contact with iodine. If the saturation with iodine had caused 

 the conversion of even a small portion of it into starch, the 

 behaviour must have been essentially different. 



The same conclusion, that the blue colouring of cellulose does 

 not depend upon the previous formation of starch, may be drawn 

 from the phsenomena which are exhibited by cellulose under the 

 action of chloride of zinc and iodine. I used for these experi- 

 ments, on the one hand, tissues which had been purified by the 

 known method of Schulze, by boiling with a mixture of nitric 

 acid and chlorate of potash, — on the other hand, cellulose in an 

 amorphous condition, which had been prepared by my colleague 

 Schlossberger by precipitating a solution of cotton in ammo- 

 niacal solution of copper by adding common salt. The reaction 

 of both modifications with iodine was perfectly identical. The 

 cellulose, brought in a dry condition into contact with the viscid 

 solution of chloride of zinc, absorbed from it so little water that 

 it did not perceptibly swell up with it*. At the same time 

 there was no trace of blue colouring. The colour is always some 

 tint of red in wood-cells and vessels, in cellulose precipitated 

 from ammoniated oxide of copper more purple or brown-red, 

 approaching more to violet in parenchymatous cells. If the 

 preparation saturated with iodine is now placed in water, the 

 colour turns suddenly to blue, and very often, especially in 

 the precipitated cellulose, to the finest indigo-blue. Yet the 

 iodine is not firmly retained under these circumstances, but is 

 washed out by water just as rapidly as out of cellulose with 

 which the iodine has been incorporated by saturating with 

 the tincture. If the chloride of zinc had caused a chemical 



* This will be seen most clearly from the following experiments. It is 

 well known that the leaves of many mosses roll up when dried, and spread 

 out again when they absorb water. I laid stems of Bartramia pomiformis 

 thus dried in the solution of chloride of zinc; but at the end of three days 

 their leaves were as strongly rolled up as at first, although they had become 

 somewhat more transparent ; the membrane of their cells had therefore 

 absorbed a smaller quantity of water than it contained in the natural con- 

 dition of the fresh plant. When I transferred the plants from the solution 

 of chloride of zinc into water, their leaves unfolded immediately. 



