THE CONDENSING PUMP 289 



If the tube is provided with fixed caps, the falling 

 bodies must evidently be placed inside before the whole 

 is finally closed. Such an apparatus is inverted by the 

 hand in order to let the bodies fall, and the experiment 

 may therefore be repeated as often as desired ; but it has 

 the disadvantage that the bodies are apt to slide down 

 along the walls of the tube instead of falling freely. 



The disc of lead with steel axis represented in fig. 

 41, p. 45, will spin about twice as long under the re- 

 ceiver, as in air. It may be set to spin in air, and 

 placed with the watch-glass upon the plate; after 

 covering it over with the receiver, the exhaustion 

 should proceed as rapidly as possible. 



The air-pump will be used, further on, in several 

 experiments on heat and electricity. 



An air-pump with an exhausting stop-cock may at 

 mce be converted into a condensing pump. If the stop- 

 cock, while the piston is drawn out, have the position 

 I. (fig. 187), and the position III. when the piston 

 s moved inwards, as if it were intended to exhaust 

 receiver upon the plate, the air enters from above 

 through ), and is ejected through a ; it follows that 

 * a closed space be connected with the tube a, the air in 

 will be condensed. One end of a piece of good sound 

 idia-rubber tubing, about 10 cm long, l cm wide, and the 

 alls having a thickness of 6 or 8 mm , is firmly tied round 

 le tube a by strong twine; the other end of the tube is 

 osed by winding twine round it and tying it as firmly 

 possible. By now using the pump as a condenser, 

 e india-rubber tube may be distended to a bag nearly 

 > cm long, and 3 or 4 cm wide, and the walls will become 

 uirly translucent. Bad india-rubber bursts during 



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