Iodide of Starch by means of Heat. 171 



It is evident from the preceding details that, during the 

 heating of the tube containing the so-called iodide of starch, 

 the solvent power of the liquid for iodine increases with the 

 temperature, and if the tube be watched at the moment when 

 the last trace of colour disappears, the tint of iodine in solution 

 becomes apparent ; but this aspect soon disappears, not only 

 on account of the small quantity of iodine present and the 

 rapidly increasing solvent power of the liquid as the tem- 

 perature rises, but also from the loss of iodine in the steam 

 which escapes from the mouth of the tube. This view of the 

 case is supported by the fact that if boiling solutions of starch 

 and iodine be mingled no colour is produced ; for at this high 

 temperature the starch and the iodine seem to be dissociated 

 and incapable of mutual action. This appears to be what is 

 meant by Briicke (as stated in the Chemical Society's Ab- 

 stracts for 1884, p. 576), who makes the bleaching to depend 

 on the affinity of warm water for iodine being greater than 

 that of cold water. 



Thus far the reasoning seems to be justified by the facts ; 

 but the restoration of the colour on the addition of chlorine, 

 apparently after all colour had been destroyed, remains to be 

 accounted for. In such case the small quantity of iodine that 

 remains after the boiling seems to be disguised under the form 

 of hydriodic acid, formed partly, as Thenard suggested, at the 

 expense of the starch ; and the action of the chlorine is to set 

 free a minute quantity of iodine, sufficient, however, for the 

 starch to restore the blue colour. The fact just given, that 

 boiling solutions of the starch and iodine may be mingled 

 without the production of colour further supports the above 

 view ; nor does the colour appear when the tube has become 

 cold ; but the addition of a few drops of chlorine solution 

 immediately starts the colour, or if to any of the tubes con- 

 taining the cold bleached solution a few drops of nitrate-of- 

 silver solution be added, a faint indication of the presence of 

 hydriodic acid is obtained. 



The conclusions arrived at seem to be: — 



1. That the blue colour disappears in consequence of the 

 dissociation of the iodine and the starch under the action of 

 a high temperature. 



2. That when the liquid is apparently permanently bleached, 

 and yet the colour is restored by the action of chlorine, a 

 minute quantity of iodine is present under the form of 

 hydriodic acid. 



the effects to vary with different kinds of starch, the blue colour dis- 

 appearing more quickly in the case of rice- than with potato-starch. 

 Casein (cheese, for example) causes a precipitation of the blue compound 

 upon itself, leaving the liquid clear above. Milk also bleaches, apparently 

 from the presence of casein. 



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