84 VEGETABLE SUBSTANCES. 



masses of nuclein are characterized by the blue 

 color they assume, which was discovered by 

 Hartig. 1 



STARCH 2 



Is colored blue by the tincture of iodine, iodide of 

 potassium, and other substances which contain 

 free iodine, since, although the so-called starch 

 cellulose cannot take up this element, granulose 

 does so. The blue color which results is not due 

 to a chemical compound, but is rather to be con- 

 sidered as the result of a (molecular ?) solution of 

 the iodine in granulose. 3 In this connection we 

 must remind the reader that the presence of water 

 is a conditio sine qua non for the reaction. Dry 

 starch treated with the vapor of iodine, or with 

 iodine in anhydrous alcohol or chloroform, is only 

 colored brown like pure starch cellulose. 



All substances which combine directly with 

 iodine destroy the blue color. Gentle warming in 

 water has the same effect ; but in cooling, the 

 starch resumes its color. 



When slowly heated in water starch grains swell 

 considerably, usually after the temperature has 

 reached 50 C. In this way starch paste is formed, 



1 Hartig: Bot. Zeitung, 1854. 



2 Nageli : Beitrage zur naheren Kenntniss der Stark egruppe, 1874. 



3 The compound resulting from the long treatment of starch with salt- 

 solutions containing an excess of free iodine is somewhat different. After 

 being raised to a red heat, this still contains about three per cent, of iodine. 

 Cf. E. Sonstadt : Note on the compound of starch with iodine. Chemi- 

 cal News, 1873, Vol. XXVIII., p. 248. 



