HANDLING SMALL AMOUNTS OF MATERIAL: FILTRATION 285 
ever so small, are to be regarded as unsatisfactory. In this 
category must be placed the ingenious filtering device of 
Haushofer,^ for it is too cumbersome, complicated, requires 
too much time, and necessitates the transferring of the solu¬ 
tion from the slide to the filtering apparatus, and back again to 
a slide. 
There are at present several practical and convenient methods 
for filtering small volumes of liquid, all based upon drawing the 
liquid through a tiny bit of filter paper held at one end by a glass 
tube of small or capillary bore while suction is applied at the 
other. The fundamental differences lie chiefly in the manner of 
applying the filtering material. 
The simplest, quickest and most useful method is that of 
Behrens .2 A filtering tube is prepared. Fig. 149, consisting of a 
glass tube F about 6o millimeters long, and of 1.5 to 2 millimeters 
bore, with walls about i millimeter thick. One end is ground 
smooth and exactly at right angles to the axis; the other end is 
rounded so as to permit the easy attachment of a small piece of 
rubber tube R, about 80 millimeters long, carrying a piece of 
glass tube M for a mouthpiece. 
The preparation of the filter and the operation of filtering a 
liquid is performed as follows; A square piece of thick soft filter 
paper P of close texture is cut slightly larger than the diameter 
of the tube, and is placed on the slide S (which lies horizontally 
on the table) close to the drop D to be filtered; the ground end of 
the tube is pressed firmly against the filter paper near one edge; 
the whole is then moved slowly into the drop; as soon as the 
paper is wet, gentle suction is applied to the upper end of the tube 
by the mouth, through the agency of the rubber tube. At the 
same time the filter paper is slowly advanced still further into 
the drop, the precipitate unless exceedingly fine will be pushed 
along in a ridge before the advancing paper and the liquid will 
rise in the tube. Care must now be taken to keep the rubber 
tube slightly curved, as shown in the cut. As soon as sufficient 
liquid has risen into the glass tube, suction is discontinued, the 
^ Haushofer, Mikroskopische Reactionen, Braunschweig, 1885, p. 160. 
* Behrens, Anleitung Mikrochem. Anal., p. 22. 
