THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 299 
I. A drop of a solution of the reagent is allowed to flow into a 
drop of the solution of the material to he 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. 
