248 H. von Mohl on the supposed Existence 



organic substance is connected with the absorption of water ; and 

 hence one might be led to find in the capacity of expansion 

 itself a reason for the blue colouring. But this conclusion 

 would certainly not be justified ; for we frequently find that the 

 capability of becoming coloured with iodine does not increase in 

 the same proportion as the capacity for swelling in water, and 

 that, on the contrary, many parts of plants, composed of hydrate 

 of carbon, swelling up strongly in water, are totally destitute of 

 the power of assuming a colour with iodine. On the other 

 hand, a certain degree of absorption of water and expansion is a 

 necessary condition for the absorption of iodine and coloration 

 by it, both with starch and with cellulose, as is simply proved 

 by the circumstance that they remain uncoloured when treated 

 in a dry condition with tincture of iodine prepared with absolute 

 alcohol. Now a substance like starch, when placed, in its na- 

 tural condition, in contact with water, absorbs such a quantity 

 as allows it to assume a determinate colour with iodine, while 

 another substance must be brought to absorb this water by the 

 simultaneous action of another means, which absorption of water 

 becomes visible to the eye, first of all by enlargement of the 

 volume of the substance, and, with simultaneous action of iodine, 

 by the colour which the latter produces. We are not justified 

 in drawing at once from such an exaltation of the expansive pro- 

 perty, and the simultaneously arising alteration of colour by 

 iodine, the conclusion that they depend upon essential chemical 

 changes in the substance, until it has been proved that the other 

 properties of the substance have been altered. This is unproved, 

 more particularly as regards cellulose which has been saturated 

 with tincture of iodine, dried, and re-softened in water ; neither 

 is it proved of cellulose which has been made to swell up in the 

 ammoniacal solution of copper; on the contrary, this may be 

 perfectly dissolved in the said fluid, and precipitated in an 

 amorphous condition from the solution, retaining all its chemical 

 properties. We see then, on the one hand, that cellulose, with- 

 out alteration of any chemical properties, absorbs more water 

 under the influence of a material which causes it to swell up 

 than it does in a natural condition, the power of absorbing iodine 

 being simultaneously increased, so that it exhibits the same blue 

 colour which the far more hygroscopic starch exhibits in its 

 natural condition when treated with solution of iodine ; further, 

 starch, on the other hand, when placed in circumstances where 

 it cannot become perfectly saturated with water, exhibits the 

 same colour as cellulose only slightly expanded in water. These 

 circumstances undoubtedly furnish proof that the blue or the 

 red colour is not dependent upon chemical diversity of cellulose 

 and starch, but that particular hydratcd conditions of both com- 



