HANDLING SMALL AMOUNTS OF MATERIAL: DECANTATION 279 



tates or fragments by drawing off the supernatant liquid, with- 

 out being obliged to resort to the longer and more tedious 

 methods of filtration. 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 rubbing a little wet " sapolio" on the 

 slide and wiping it dry with a clean cloth will materially improve 

 the surface. The other requisites for successful decantation 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. 146. 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 



