EFFLUX OF GASES 301 



being introduced into the glass tube, and be removed again and put 

 aside after having been used ; for if allowed to become dry in the 

 tube, the india-rubber adheres too closely to the glass, and cannot 

 be moved afterwards without injury. 



A forcing-pump with one barrel and an air-vessel, fig. 199 A, 

 may also be constructed without difficulty. The barrel is again 

 made of a glass tube, and the suction-tube is connected with it in 

 the same manner, the brass collar having, however, in this apparatus, 

 an aperture in the side into which a horizontal tube is fixed, so as 

 to project somewhat inside the wider tube and to fit tightly in the 

 aperture ; no wire will then be required for holding it in position 

 while it is soldered. This lateral tube, before the valve is intro- 

 duced, also receives a lining of sealing-wax at the edge for fixing the 

 grlass tube which leads to the air-vessel. The latter is made of a 

 wide-mouthed bottle, such as is used for pomatum, for which a 

 ?ood tight-fitting cork is selected ; the cork is bored with two 

 boles. The upper end of the glass tube which connects the 

 lir-vessel with the pump-barrel should be carefully ground and 

 melted so that it may have a perfectly flat and smooth edge ; 

 t should project above the cork about O mm '5. The valve, made 

 )f oiled silk or india-rubber, is fixed by means of a strip of zinc 

 )f the form shown in fig. 199 B, of which the straight por- 

 :ion is stuck into the cork. Another glass tube, bent at right 

 ingles, passes also through the cork, and is connected by a piece of 

 ndia-rubber tubing with a glass tube ending in a fine aperture, 

 hrough which the jet issues. The india-rubber tube is tied round 

 roth glass tubes, or else it might be forced off by the pressure of 

 he water. The piston in this pump is also made of a piece of india- 

 ubber tube, but it is tied in the middle, so as to be wider at each end ; 

 fc prevents in this form the passage of water in both directions. The 

 mall board to which the pump is attached should be clamped to the 

 able, so as to have both hands free ; one for working the pump, the 

 ither for directing the jet. 



Both pumps may properly be closed at the top by corks, which 

 erve as guides for the piston-rods ; the holes in the corks should be 

 ) wide as to permit the rods to move in them easily and without 

 ;rks. 



An india-rubber tube drawn over the lower metal tube may serve 

 :* suction- tube instead of a glass one ; but it should be firmly tied 

 ) the metal tube, or it will not close air-tight at the joint. 



30. The Reaction of Efflux in Gases. Phenomena of 

 Auction. When gaseous bodies flow through the aper- 



