238 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. 126. 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 



