310 ELEMENTARY CHEMICAL MICROSCOPY 



ing or coloring can be quickly and simply accomplished, advan- 

 tage should at once be taken of the fact. 



As examples of qualitative tests which may be considered as 

 falling under this method, the following may be cited: 



In testing for perchlorates, the addition of a permanganate 

 will yield colored perchlorate crystals. 



Iodine and bromine are revealed by their coloring starch 

 granules, or the presence of a compound setting free iodine from 

 an iodide or from an iodate is ascertained by starch. Or, on the 

 other hand, starch is easily differentiated from other substances 

 by staining with an iodine solution. 



Most oil or fat globules may be stained by alkanin. 



Fullers earth affords a simple means of distinguishing between 

 vegetable and aniline dyes and in a few cases between certain 

 aniline dyes themselves. 



In the microchemical examinations of rock sections, aluminum 

 hydroxide can be stained with congo red and gelatinous silica 

 with malachite green — tests which may be employed in testing 

 for "weathering," etc. 



EXPERIMENTS. 



a. Next to a drop of a dilute solution of HC10 4 or NH 4 C10 4 , place a drop 

 of RbCl solution (or KC1, if no Rb is obtainable). Cause the Rb to flow into 

 the perchlorate (Method 7). In a few seconds colorless, characteristic crystals of 

 RbC10 4 separate. Place a drop of dilute KMn0 4 next to the preparation and 

 cause it to flow into it. The crystals of RbC10 4 will become colored pink. The 

 resulting compound is a solid solution (isomorphous mixture) of the permanganate 

 in the perchlorate, due to adsorption. 



b. To a drop of a dilute KI solution add a few granules of potato or arrow-root 

 starch. Stir. Examine under the microscope. Add at the center a very minute 

 fragment of pure KN0 2 or NaN0 2 . Examine again. The starch granules should 

 appear at the most only very slightly colored. Add a trace of very dilute HC 2 H 3 2 

 or H 2 S0 4 . The starch granules turn blue or purple, due to adsorption of liberated 

 iodine. 



Repeat the experiment, substituting a bromide for the iodide and (NH 4 ) 2 S 2 8 for 

 the KN0 2 



X. The reagent dissolved in a volatile solvent is spread in a film 

 upon an object slide in such a manner as to yield a coating or varnish 

 non-crystalline in character, and across this prepared surface a 

 solution of the unknown material is drawn. 



