PHENOMENA OF SUCTION. 311 



held in the retort stand, and offers, besides, less resistance by 

 friction to the passage of the water through it than a narrower tube ; 

 the velocity of the water hence becomes greater. If a reservoir is 

 used, the glass tube may be left straight, 8 or 10 cm long, and in 

 order to connect it firmly with the india-rubber tube which leads 

 from the reservoir, a short piece of a wider glass tube, which 

 corresponds to the width of the rubber hose, should be fixed with 

 sealing-wax over the end of the straight glass tube. 



A capacious vessel (a basin) is placed near the mouth of the tube 



for the reception of the discharged water. The narrow zinc tube is 



dipped in water contained in a saucer and coloured red by magenta. 



The supply of wat<- i being provided for, if necessary with the help 



of an assistant, the finger is placed at a as soon as the liquid begins 



! to flow out, and held over the aperture until the air is squeezed 



! from the apparatus, and has escaped from the zinc tube. The 



finger is then removed from a. A full jet will i? ue from the 



: orifice, which will be reddened by the admixture of the water from 



; the saucer, and if the discharge be maintained for some time, the 



saucer will be completely emptied. The finger should not close the 



aperture a but only render it narrower ; if a be closed the air would 



not be removed, and the water would flow into the saucer. 



A current of air which flows from a narrower tube 

 into a wider one, produces suction in a similar manner 

 as a current of water. If air be strongly blown into the 

 narrower tube of the contrivance represented in fig. 202 

 E, at w, or in fig. 203 ., water will be made to ascend 

 in the vertical tube and will be thrown out at a in small 

 drops. 



The effect of suction produced by a current of air is 

 rendered especially obvious, if the current is allowed 

 to expand between two flat discs, such as those in the 



i little apparatus, fig. 204 A. A circular disc of card- 

 board, 10 cm in diameter has a hole in the middle. A 



' glass tube, about 8 mm wide, bent at right angles, is passed 

 through a cork which is glued upon the disc, so that 

 the bore of the tube is exactly over the hole in the 

 disc. A second disc, of stout paper, or thin cardboard, 



