8 INTRODUCTION. 



29. Iodine. This element is only ^'ery slightly soluble in 

 pure water. Upon exposure to strong light, however, a some- 

 what larger amount of iodine passes into solution after a while, 

 owing probably to formation of hydriodic acid. If it is neces- 

 sary to examine the effect of iodine alone, as in certain parts of 

 Lichens, a fresh solution should be used. In fact, it is recom- 

 mended that in such cases a minute fragment of solid iodine be 

 placed in pure water under the cover-glass at the mom.ent of 

 examination. 



30. But for all ordinary examinations, a solution of iodine in 

 water which contains iodide of potassium is used. The propor- 

 tions employed vary widely. A convenient strength is obtained 

 hy dissolving one gram of iodine and five grams of potassic 

 iodide in enough water to make one hundred cubic centimeters. 

 Even this solution is too strong for some purposes. In a few 

 cases a different solution is advised, made hy dissolving five 

 centigrams of iodine and twentj* centigrams of potassic iodide in 

 fifteen grams of water.^ But, in general, dilute solutions are 

 preferable. 



31. A solution of iodine and iodide of potassium in glyce- 

 rin is employed by some. An alcoholic solution is sometimes 

 useful. 



32. Iodine is a characteristic test for starch, to which it 

 imparts a blue color, depending for its depth ehiefl_v upon the 

 strength of tlie solution. Iodine in absolute alcohol gives with 

 Ayy starch a brownish color ; if the alcohol is not absolute, that 

 is, anhjdrous, a blue color is given as with ordinary aqueous 

 solutions. 



33. In most cases cellulose is colored pale jellow to deep 

 brown by iodine. If the specimen is acted on by concentrated 

 sulphuric acid, either just before or just after the application 

 of the iodine, a blue color appears. This reaction for cellulose 

 is disguised by various incrusting matters, which can be removed 

 by strong acids or alkalies ; after their j'emoval the washed 

 specimen will give the characteristic cellulose reaction (see also 

 143). 



34. Iodine and a metallic iodide in a strong solution of chlo- 

 ride of zinc form a very useful reagent for cellulose, to which a 

 blue color is given. The reagent is easily made bj- dissolving 

 pure zinc in concentrated hjdrochloric acid until there is no 

 further action of the acid. The solution, with a little metallic 



1 Poulseii. 



