AIR-PUMP. 171 



haustion of small receivers. The liquid used is mercury. 

 The vertical pipe, below the arm t (Fig. 87), must be 

 longer than the barometer column (six feet is a common 

 length), and have a diameter of not more than -fa inch. 

 The mercury is admitted by large drops, which, filling 

 the pipe, act as valves and in their fall force out succes- 

 sive quantities of air before them. 



(a.) With such an instrument, it requires about half an hour to 

 exhaust a half liter receiver, but the average result attainable is a 

 tension of about one-millionth atmosphere or 0.00003 inch of mer- 

 cury. By this means a tension of only T^Tnhnnr atmosphere has 

 been secured. The mercury acts as a dry, frictionless, perfectly 

 fitting, self-adjusting piston. Special precautions must be taken to 

 make the connection air-tight. The only work of the operator is to 

 carry the mercury from the cistern at the foot of the fall tube to 

 the funnel at the top. 



291. Bimsen's Air-Pump. In Bunsen's air- 

 pump the principle is the same, but the liquid -used is 

 water, and the length of the vertical pipe at least thirty- 

 four feet. Such an air-pump may be easily provided in a 

 laboratory where the waste-pipe of the sink has the neces- 

 sary vertical height. The tube t (see Fig. 87) being con- 

 nected with the receiver, has its free end inserted in the 

 waste-pipe a little way below the sink. A stream of water 

 properly regulated, flowing into the sink, completes the 

 apparatus. 



292. The Condenser. Tlie condenser is an 

 instrument for compressing a large amount of air 

 into a closed vessel. It differs from the air-pump, 

 chiefly, in that its valves open toward the receiver. 

 The cylinder is generally attached directly to the stop- 

 cock of the receiver. Its operation will be readily un- 

 derstood. Sometimes the upper valve, v f , instead of 



