APPLICATIONS OF MARIOTTE S LAW. 



257 



upper end; when full, the upper end is closed and 

 the pipette withdrawn ; but the liquid will not run out, 

 for the air which may have remained above the liquid, 

 though it has at first the same pressure as the air 

 outside, expands and allows a small portion of the 

 liquid to escape : its pressure thus diminishes until the 

 excess of the external pressure of the air over the 

 internal is equal to the pressure of the liquid in the 

 pipette. 



By means of a pipette with a somewhat capacious bulb, so that the 

 \ r olume of the enclosed air is not too small, it may be shown that the 

 iir inside expands in consequence of the pressure of the external air 

 oeing partially neutralised by the pressure of the liquid in the 



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i 



FIG. 178 ( real size}. 



pette which acts in an opposite direction. Thus suppose that, after 

 .rtly filling the pipette, the water has been allowed to run out 

 itil a column of 20 cm remains behind, as shown in fig. 178 A. At 

 the pressure of the atmosphere is equal to a column of water 10 



