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 

 Ffaushofer, 1 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 60 millimeters long, and of 1.5 to 2 millimeters 

 bore, with walls about 1 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 



1 Haushofer, Mikroskopische Reactionen, Braunschweig, 1885, p. 160. 



2 Behrens, Anleitung Mikrochem. Anal., p. 22. 



