8 MICRO-CHEMICAL REAGENTS. 



other preparations are removed from water into a 

 concentrated alcoholic tincture of iodine, small, 

 black, rhombic crystals of iodine often make their 

 appearance. 



CHLOR-IODIDE OF ZINC. 



In the preceding section mention has been made 

 of a solution of iodine in chloride of zinc. 1 We 

 may now consider this a little more fully. 



This preparation is made by dissolving zinc in 

 pure hydrochloric acid, evaporating the solution to 

 the density of sulphuric acid, in contact with 

 metallic zinc, and adding as much potassic iodide 

 as the solution will take up. Finally, it is saturated 

 with metallic iodine. 2 The color of the reagent 

 should be reddish-brown ; it should have the odor 

 of iodine, and small crystals of pure iodine should 

 precipitate with time. As a precaution against 

 the formation of hydriodic acid, it should be kept 

 in the dark, although this is less important than 

 with the other iodine preparations. 



Chlor-iodide of zinc is especially useful for the 

 detection of pure cellulose, since the zinc chloride 



1 Nageli: Verhalten der Zellhaut zum Jod. Sitzungsber. d bayr. 

 Akad. der Wiss., 1863, p. 383; Das Mikroskop, 1877, p. 474. Mohl: Blaue 

 Farbung der Vegetab. Zellmembran durch Jod. Flora, 1840. See also the 

 works cited above. 



2 The directions of Gronland, Cornu and Rivet (Des Preparations 

 Microscopiques, Paris, 1872, p. 75), are incorrect, since the most important 

 element the iodine is not present. 



