APPLICATIONS OF MARIOTTE'S LAW. 259 



below the handle there is a small aperture a. When liquid is poured 

 into the funnel while the lower aperture is closed by the finger, 

 it ascends into the space between the two funnels because this space 

 communicates with the inner funnel by the open aperture of the 

 latter. When the funnel is full, the finger which closes the lower 

 end is withdrawn but the aperture a is closed, which may be done 

 without being perceived by a finger of the hand which holds the 

 funnel ; the funnel will then apparently empty itself, Avhile really a 

 quantity of liquid remains behind as long as a is kept closed. 



If the lower aperture of a pipette is very wide, it is 

 impossible to retain the liquid in it, because in that 

 case the air and the water have sufficient space to pass 

 i each other ; but if the aperture be dipped in water, no 

 air can enter, and hence no liquid escapes. Similarly 

 when a vessel which is filled with liquid is turned 

 upside down and its mouth is immersed in an additional 

 quantity of liquid, the vessel remains full, the liquid 

 being kept in it by the external pressure of the atmo- 

 sphere. 



This principle may be applied when it is desired to fill a small 

 vessel repeatedly with liquid from a larger one, and to save the 

 trouble of constantly attending to it, as for example when a liquid 

 lias to be filtered in order to remove substances suspended in it. 

 The bottle containing the liquid is inverted, its mouth being closed 

 with the finger or a circular piece of stiff paper, and clamped in the 

 retort stand so that the mouth of the bottle dips a few millimetres 

 into the filter. When the finger or the paper is removed the liquid 

 Hows into the filter until the mouth of the bottle is immersed; ths 

 How then ceases, but whenever so much liquid has passed through 

 (the filter as to make the surface sink lower than the mouth of the 

 [bottle, air ascends in the latter, and fresh liquid flows out. Fig. 

 '180 shows such an arrangement without the necessary supports for 

 jhe bottle and the funnel, 



. A plain filter is made from a circular piece of blotting paper, 

 vhich is folded into halves and then into quarters ; it is then opened 

 >ut, leaving three thicknesses on one side and one thickness on the 

 >ther, so as to form a smooth cone, which is carefully fitted into a 

 'unnel in such a manner as to be well supported all round. 



