METHODS FOR HANDLING SMALL AMOUNTS OF MATERIAL 231 



tates or fragments by drawing off the supernatant liquid, without 

 being obliged to resort to the longer and more tedious methods 

 of nitration. Success in drawing off a liquid requires, in the 

 first place, a perfectly clean slide free from grease, otherwise the 

 liquid will not flow properly; and, secondly, patience, care and 

 a steady hand. The first requirement is met by treating the 

 slides in one of the usual cleaning mixtures of which the chromic- 

 sulphuric acid is the best, and subsequently thoroughly washing 

 them. Sometimes placing a drop or two of ammonium hydroxide 

 on the slide and wiping it dry with a clean cloth will materially 

 improve the surface. The other requisites for successful decan- 

 tation are dependent upon the manipulative ability of the analyst 

 and may be acquired only by practice. 



Although the phrase synonymous with decantation draw- 

 ing-off is self-explanatory and the method is quite obvious, 

 there are, nevertheless, several points upon which the success of 

 the operation depends. 



Assuming that the drop of liquid is situated, as usual, at the 

 corner of the slide, the operator proceeds as follows : The slide is 

 held in a horizontal position ; the end of a drawn-out glass rod or 

 a platinum wire is carefully introduced into the edge of the 

 drop and is then slowly drawn across the slide (the slide being 

 simultaneously slightly inclined in the same direction) until a 

 distance of about one centimeter is reached. If the slide is per- 

 fectly clean the liquid will follow the rod or wire in a narrow 

 stream. A circular motion is now given the rod, resulting in the 

 spreading out of the little stream into a drop ; this induces a flow 

 of the liquid from the original drop. The steps in the decanta- 

 tion are indicated in Fig. 123. The flow is aided by increasing 

 the angle of inclination of the slide, providing, of course, there is no 

 tendency on the part of the sediment to flow with the liquid. The 

 important points, which can be learned only by practice, are the 

 proper angle and the rate and manner of spreading out the drop. 

 Should there be any tendency of the sediment to pass over with 

 the liquid, reduce the angle at once. If the sediment tends to 

 form a dam and prevent the passage of the clear liquid, it is neces- 

 sary to start a new current at one side of the barrier or to break 



