286 
ELEMENTARY CHEMICAL MICROSCOPY 
rubber tube compressed at its upper end between the fingers and 
is simultaneously straightened to prevent the forcing out of the 
liquid. To lift the tube from the slide and the piece of filter 
paper, stretch the rubber tube very gently and raise the whole 
apparatus. The filtrate contained in the tube is removed by 
bringing the ground end in contact with a slide and bending the 
rubber tube, the upper end of which is kept closed; the liquid 
will generally flow out at once; if not, straighten the tube, open 
the upper end and blow very gently, but only just sufficiently to 
expel the drops. 
Fig. 149. Behrens Method of Filtration. 
A little practice is required in order to apply the proper pressure 
of the glass tube upon the filter paper and to maintain this 
pressure uniformly without tipping the tube out of its vertical 
position. 
The chief difficulties encountered in rapid work are: (i) The 
danger of carrying the filtrate up into the mouth or into the 
rubber tube by air bubbles, which are always drawn into the tube 
when the liquid to be filtered has all been absorbed by the filter 
paper and sucked into the tube, and (2), it not infrequently 
happens that the filtered liquid begins to flow out when suction 
