IODINE. 5 



treated with strong sulphuric or phosphoric acid, or 

 with an aqueous solution of zinc chloride, it colors 

 blue when iodine is added, these reagents convert- 

 ing the cellulose into amyloid, I a carbo-hydrate 

 related to starch. Hydriodic acid also acts in the 

 same way, and hence old solutions of iodine some- 

 times color the cell-wall blue, when this acid has 

 formed through the action of the water or alcohol 

 of the solution. Cellulose membranes that have 

 been allowed to dry after treatment with a solu- 

 tion of iodine, and are again moistened with water, 

 often turn blue, the organic matter having in- 

 duced the formation of hydriodic acid. The epi- 

 dermal cell-walls of many seeds and pericarps that 

 swell into mucilaginous masses when moistened, as- 

 sume a blue color with iodine-water only after the 

 swelling has reached a certain stage. Not unfre- 

 quently a membrane lying in an iodine solution 

 passes through several shades of color in the course 

 of a longer or shorter time. Most reagents that 

 cause cellulose to turn blue when used in combina- 

 tion with iodine, also cause it to swell, approxi- 

 mately, in the following ascending order : Potassic 

 iodide, iodide of zinc, nitric acid, phosphoric acid, 

 potassic hydrate, hydriodic acid, and sulphuric 

 acid. 



We have thus an excellent test for pure cellu- 

 lose in iodine and any amyloid-producing reagent. 



1 Not to be confounded with the substance known to animal histolo- 

 gists by the same name. 



