256 



APPLICATIONS OF THE SIPHON. 



the same level inside and outside, provided that in filling the tube 

 by suction no air-bubbles are left in its upper portion. A small 

 cylindrical float of stearine, which must be narrow enough to move 

 up and down the tube without friction, shows the height of the water 

 in the reservoir more distinctly ; if the wall of the room is dark, the 

 float may be left white ; but if the wall is of a light colour, the 

 float should be blackened by rubbing it over with powdered graphite 

 (black-lead). 



The action of the pipette, of which we have made use 

 on a previous occasion (fig. 11), and of the 'wine- 

 taster/ fig. 177, used in the wine trade for removing 

 small quantities of wine from casks, depends upon 

 atmospheric pressure. If the lower end of a pipette 

 is dipped into a liquid, the effect of suction at the 





FIG. 176 (^ real size). 



FIG. 177 ( real size). 



upper end is to diminish the pressure within; the 

 liquid is therefore pressed upwards by the excess ol 

 the external over the internal pressure, it rises in 

 the pipette and cannot flow back again if the upper 

 end is closed by the finger, and the air is thus pre- 

 vented from entering. The pipette may also be filled 

 by simply dipping it into the liquid without closing the 



