Iodide of Starch by means of Heat. 169 



(1871, p. 136) that " the cause of this change of colour is not 

 known." 



Some writers, as quoted by Gmelin, maintain that if the 

 iodine which evaporates in boiling be expelled by blowing air 

 into the vessel, the iodide of starch remains colourless after 

 cooling, but if it can reabsorb the iodine-vapours on cooling 

 the colour is restored. A sufficient answer to this is that the 

 colour begins to reappear at the bottom of the tube long before 

 the vapours at the upper part can possibly be reabsorbed. The 

 vapours may even be blown away and yet the colour reappear. 



The usual mode of accounting for the discoloration is that 

 given in Watts's i Dictionary of Chemistry/ under Starch, 

 v. 1868, p. 410: — "The liquid may be decolorized by ebulli- 

 tion, whereby the iodine is volatilized ; if, however, the boiling 

 be not continued for a sufficient time to volatilize the whole of 

 the iodine, the blue colour reappears as the liquid cools." 



This statement is but partially true ; for the colour disappears 

 long before the boiling-point is reached, and it does not account 

 for the fact that when the liquid is apparently permanently 

 bleached and allowed to get cold, the addition of a few drops 

 of solution of chlorine will restore the blue colour. 



In examining this subject experimentally, the difficulty 

 seemed to lie in the multitude of explanations rather than in 

 any inherent difficulty belonging to it. My first care was to 

 determine the temperature at which the bleaching takes place. 

 For this purpose, fifteen grains of iodine were treated with 

 half-a-pint of distilled water, and five grains of each of four 

 varieties of starch were rubbed up and then boiled with about 

 an ounce of distilled water. 



1. One ounce of solution of starch crisp from maize was 

 mixed with one ounce of the iodine solution in a small globular 

 flask and heated over the flame of a spirit-lamp. The colour 

 entirely disappeared at from 150° to 160° Fahr. (65° to 71° C). 

 Left to cool in the air of the room the colour began to return 

 at 120° F. (49° C), and the full colour was restored at 70° F. 

 (21° C). The flask was heated a second and a third time, with 

 nearly similar results. 



2. A solution with rice-starch became paler at 105°, much 

 paler at 115°, pale at 120°, and the colour entirely disappeared 

 at from 135° to 140°. The flask was plunged into cold water, 

 when the solution became reddish at 135°, purple at 100°, 

 violet at 80°, and blue-violet at 70°. 



3. With sago-starch the colour became lighter at 110°, pale 

 blue at 120°, pale at 140°, and was bleached at 150°. In cold 

 water the colour returned at 130°, at 85° it was full blue, and 

 at 80° quite opaque, like ink. 



4. The mixture with potato-starch was of a blue-black 



