302 ELEMENTARY CHEMICAL MICROSCOPY 



777 A . A tiny drop of the reagent is added directly to the test drop 

 at its center. 



This procedure is effective in all cases where the crystalline 

 phase, which is wished, is not too slowly formed, has great crystal- 

 lizing powers and forms a large molecule. It may be said, that, 

 in a general way, the addition of a drop of the reagent directly 

 to the drop to be tested is applicable to practically all micro- 

 chemical reactions. But in many special cases the crystals 

 separating are not as characteristic nor as constant in their habit 

 as in other methods, nor does the reaction take place with suffi- 

 cient rapidity. 



The direct addition of the reagent is also practiced when 

 a heavy agglutinated precipitate results, which must subse- 

 quently be freed from its supernatant liquid and then recrys- 

 tallized. 



The most frequent cases where reagent drops are added are 

 in acidification, alkalinization, neutralization; and in the addi- 

 tion of some reagent whose purpose is to mitigate the dele- 

 terious action of some compound present, as, for example, the 

 addition of sodium or ammonium acetate to prevent a free mineral 

 acid from interfering with a test. Usually, however, a fragment 

 of the solid acetate is added rather than a drop of solution. Or 

 we may add a drop of glycerine solution to retard the formation 

 of certain crystals. 



EXPERIMENTS. 



a. To a drop of a dilute solution of HgCl 2 add a fragment of KI. Note the 

 kind of crystals formed and their position with respect to the fragment of KI. 

 After the fragment of KI has dissolved leaving a clear area, add to its center a 

 tiny fragment of CuSO*; the Hgl 2 which has dissolved will be reprecipitated. 



b. To a drop of a very dilute solution of HAuCL, (chloroauric acid) add a 

 tiny fragment of T1N0 3 . In this case the characteristic crystals consisting of 

 TlAuCL, • 5 H2O (?) form upon the fragments of the reagent. 



c. To a drop of Pb(N0 3 )2 solution add a tiny drop of a dilute solution of 

 CUSO4. Stir. Add a fragment of Na(C 2 H 3 2 ), stir until almost dissolved. Now 

 add a fragment of KN0 2 and follow with a trace of dilute HC 2 H 3 2 . Tiny black 

 cubes of the triple salt 2 (KN0 2 ) • Cu(N0 2 ) 2 • Pb(N0 2 ) 2 separate. 



d. To a drop of a solution of Pb(N0 3 ) 2 add a tiny fragment of metallic mag- 

 nesium. Try in like manner a number of elements in the electrochemical series. 



