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,^ 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, 
* A convenient form of machine is shown in Fig. 90. 
