chap, xxin.] THE PIPETTE AND SYPHON. 279 



descends again the water is prevented passing back- 

 wards by a valve c, while by the opening of other 

 valves A and B it is permitted to pass through the 

 piston, and is lodged in the upper part of the barrel. 

 The re-ascent of the piston causes the piston valves 

 to close, and the water is therefore driven out through 

 the outlet tube. 



The pipette also illustrates the same principles 

 (Fig. 121). It is a glass tube blown out in the centre 

 into a bulbous portion. One end is pro- 

 longed into a fine point, the other is the 

 full diameter of the tube, and is evenly 

 ground. By applying the mouth to the 

 wide end and sucking, the air is rarefied, 

 and if the lower end be dipping in liquid, 

 the liquid rises in the tube, and into the 

 bulb. As soon as the desired quantity is 

 drawn into the bulb, the upper end is 

 quickly covered with the wet finger or 

 thumb. The air is thus prevented from 

 entering, and the pipette can be lifted out 



O' a 1 



of the fluid without any of its contents The Pipette, 

 escaping. Any desired quantity can be per- 

 mitted to escape by slightly moving the finger to permit 

 the entrance of a little air. By this means part 

 of the liquid in a vessel may be removed without 

 disturbing the remainder. 



The syphon consists of a tube open at both ends 

 but curved on itself, so as to have two limbs. It is so 

 placed that one limb dips into the liquid to be removed, 

 and the other discharges at a lower level (Fig. 122). 



By suction at the lower end the tube is first of all 

 filled with the liquid, and then under the influence of 

 atmospheric pressure, and the difference of levels, the 

 flow will continue unless air be permitted to enter, or 

 the levels become equal. 



Where the liquid to be withdrawn would be 



