424 C. F. Roberts — Blue Iodide of Star eh. 



withdrawing hydriodic acid from the molecule, thus setting- 

 free iodine. 



(<?) By a solution of a silver salt. One of the strongest 

 proofs which Mylius gives for the correctness of his views is 

 that the blue color is destroyed by a silver salt, that the addi- 

 tion of pure iodine now does not restore the color, only turn- 

 ing it yellow, but that the addition of hydriodic acid or potas- 

 sium iodide gives the blue at once. 



I have found, in fact, that a single drop of a very dilute 

 solution of silver nitrate is sufficient to destroy the deep blue 

 color in a test-tube full of liquid. A natural explanation, in 

 the light of the preceding experiments, is that the silver nitrate 

 withdraws hydriodic acid ; and the ease and completeness of 

 the change supports the view that this exists as such in the 

 molecule, since in many organic compounds, the halogen is 

 extracted only with difficulty if at all by silver nitrate. This 

 view is still further supported by the fact that the blue color 

 is immediately restored by the addition of a little hydriodic 

 acid. The color is also restored, though not to its former 

 depth, by the addition of a few drops of strong hydrochloric 

 acid, or by a larger quantity of the dilute acid, though it is 

 not affected by sulphuric acid. This can readily be explained 

 on the supposition that the silver iodide is partially decom- 

 posed by the hydrochloric acid, giving sufficient hydriodic acid 

 to form some of the blue iodide. That this is the correct ex- 

 planation rather than that hydrochloric acid can be substituted 

 for hydriodic in the starch compound without changing its 

 color is proved by filtering after adding silver nitrate, and 

 adding hydrochloric acid to the filtrate, when no blue color is 

 produced. 



If we concede that in all three of these cases, the decompo- 

 sition has been effected by a withdrawal of hydriodic acid, it 

 certainly shows a tendency for that portion of the molecule to 

 split off, which is suggestive of its existing already formed as 

 such in the molecule, and in so far is in harmony with Mylius's 

 views, though the mere fact that hydriodic acid is withdrawn 

 is no proof that it goes into the molecule as such. The only 

 portion of the above which can be considered at all as direct 

 proof of Mylius's statement is that after the blue color has 

 been destroyed by silver nitrate, it can be reformed by the 

 addition of hydriodic acid. This would seem certainly a strong 

 argument in support of Mylius. 



We pass next to the consideration of 



2. The Conditions of formation of Blue Iodide of Starch. 



Mylius makes three statements in regard to this which may 

 be quoted here. 



