AIR-PUMP. 



being placed in the piston, is placed in 

 a tube opening from the side of the cylin- 

 der below the piston. By connecting 

 this lateral tube with a reservoir contain- 

 ing any gas, the gas may be drawn from 

 the reservoir and forced into the receiver. 

 When thus made and used, the instru- 

 ment is called a transferrer (Fig. 104). 



Note. The pupil will notice that in the case 

 of the air-pump, the condenser, the transferrer, 

 and the lifting and force pumps to be subse- 

 quently considered, the valves open in the di- 

 rection in which the fluid is to move. 



FIG. 104. 293. Experiments. A person 



having an air-pump has the means of 

 performing almost numberless experiments, some amusing 

 and all instructive. Other experiments, which may be per- 

 formed without such apparatus, have been purposely de- 

 ferred until now. The pupil should explain each experiment. 



(1.) The pressure of the atmosphere, which is transmitted in all 

 directions, may be illustrated by filling a tumbler with water, plac- 

 ing a slip of thick paper over its mouth and holding it there while 

 the tumbler is inverted ; the water will be supported when the 

 hand is removed from the card. 



(2.) Plunge a small tube, or a tube having a small opening at the 

 lower end, into water, cover the upper end with the finger and lift 

 it from its bath. The water is kept in the tube by atmospheric 

 pressure. Remove the finger, and the downward pressure of the 

 atmosphere, which was previously cut off, will counterbalance the 

 upward pressure and the water will fall by its own weight. Such 

 A tube, called a pipette, is much used for transferring small quanti- 

 ties of liquids from one vessel to another. The pipette is often 

 graduated. 



(3.) The "Sucker" consists of a circular piece of thick leather 

 with a string attached to its middle. Being soaked thoroughly in 

 water it is firmly pressed upon a flat stone to drive out all air from 

 between the leather and the stone. When the string is pulled 



