380 Mr. R. W. Wood on a Duplex Mercurial Air-Pump. 



the apparatus is exhausted as completely as possible. During 

 the exhaustion it must be supported in the position shown in 

 fig. 3 ; otherwise the air escaping from the discharge-tube 

 will throw the mercury violently against the top of the bulbs. 

 It is a good plan to carefully heat the bulbs and the discharge- 

 tube by means of a Bunsen burner while the pump is in 

 action, in order to drive off moisture. The current of a fair- 

 sized induction-coil should also be passed through the dis- 

 charge-tube for a few minutes to rid the electrodes of air as 

 completely as possible. It will be found that the vacuum 

 cannot be made perfect enough to give a Crookes dark space 

 of more than an inch, owing to the leakage through the 

 rubber hose. The capillary part of P is now heated to fusion 

 in a small flame which hermetically seals the entire apparatus. 

 The comparatively poor vacuum in the discharge-tube can 

 now be made as perfect as is possible with any mercury-pump 

 by slowly rocking the apparatus, holding it by the bulbs A A. 

 If the pump is properly made, the traps hold and require no 

 attention : if not, a little dexterity is required, to prevent the 

 mercury from running out into the bulb, and they have to be 

 constantly watched. Care must of course be taken that the 



air-bubble, compressed into the trap by each stroke, is driven 

 entirely around the bend and into the reservoir B. If for 

 any reason the pump requires to be subsequently opened, it 

 must be placed in the inclined position and a file scratch 

 made on the tube P. A bit of red-hot glass pressed against 

 the scratch will cause a crack through which the air will 

 slowly enter. If the tube be broken open suddenly, the 

 mercury will be forced over into the discharge-tube. I con- 



