THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 299 



/. A drop of a solution of the reagent is allowed to flow into a 

 drop of the solution of the material to be tested. 



This method of applying the reagent is more often employed 

 than any other, and is generally far preferable to the ad- 

 dition of a drop of reagent directly to the solution to be 

 tested. 



A perfectly clean object slide is required. Upon it near a cor- 

 ner place a small drop of the solution of the material to be tested. 

 This drop should be spread out until it attains a diameter of 

 approximately 5 millimeters and a depth of not over half a milli- 

 meter. A drop of the reagent of the same diameter but about 

 twice the depth is next placed adjacent to the first drop at a dis- 

 tance of 2 to 3 millimeters. The concentration of the reagent 

 drop should usually be slightly greater than that of the substance 

 being tested. By means of a platinum wire or drawn-out glass 

 rod, a tiny channel is made to flow from the reagent into the 

 test drop, the object slide being tipped very slightly to facilitate 

 the flow, but under no condition should the two drops merge 

 completely. 



Having a higher concentration in the reagent drop usually 

 leads to a flow of this liquid at a lower level and therefore close 

 to the object slide because of a slightly greater 

 density than that of the solution of the sub- 

 stance. Crystals thus tend to form upon the 

 slide instead of floating about in the liquid. 

 The more perfect crystal faces are on the 

 upper side, or, in other words, that side most 

 easily studied by means of the microscope. 

 Crystals which float about usually grow 

 downwards from the upper surface of the 

 test drop and therefore have the well-de- 

 veloped faces on their under side, which must remain more or 

 less invisible. 



The maximum sizes of drops are shown in the diagram, Fig. 

 160. The reagent drop R has been made to flow into the drop 

 to be tested S through a tiny channel c. The crystalline phase 

 constituting the identity test separates at p. 



