HANDLING SMALL AMOUNTS OF MATERIAL: DECANTATION 281 



Liquids which have been decanted but which are not suffi- 

 ciently clear may be evaporated and treated by the method 

 described in the preceding paragraph. 



Washing precipitates by decantation may be performed by 

 drawing off the liquid as above, adding a drop of washing liquid 

 to the residue, allowing to stand for a few seconds and drawing 

 off as before. The process is repeated as long as is thought 

 necessary, or until tests applied to the decanted liquid prove that 

 the washing is sufficiently complete. It is obvious that with a 

 pure solvent, containing no compounds in solution, the simplest 

 test is evaporation to dryness and the obtaining of no perceptible 

 residue. 



In the event of a number of drops being obtained in the process 

 of washing, all of which must be saved and united for subsequent 

 examination, it is best to transfer them to a second clean slide; 

 this is done by decanting into the extreme corner of the slide, 

 cutting off the stream with filter paper and warming as already 

 described. Now slowly raise the slide to an almost vertical 

 position and bring the corner, holding the decanted drop, in 

 contact with the slide prepared to receive it. Touch the drop 

 at the corner with a drawn-out glass rod or platinum wire and 

 the drop will flow at once on to the slide below. Raise the verti- 

 cally held slide and warm its corner over the micro-flame, wash 

 the residue as before and again transfer. The united washings 

 may afterward be concentrated to the proper volume by evapo- 

 ration. 



In all cases where decantation is to be practiced the size of the 

 drop to be treated must be somewhat larger than that employed 

 in tests alone. 



Decantation by Means of the Centrifuge. — Next in impor- 

 tance to the methods above described for separating sediment 

 from liquid must be placed the centrifugal machine. 



A "two-speed" machine, with hematokrit frame, should be 

 purchased, 1 since it is seldom that sufficient liquid is available 

 in ordinary microchemical work to permit of the usual sedimen- 

 tation tubes being employed. With the hematokrit attachment, 

 1 A convenient form of machine is shown in Fig. 90. 



