142 J. A. POLLOCK. 



Sprengel pump, passes through a mercury sealed cork at K. 

 At D there is a mercury sealed tap and at H a short length 

 of small bore tube to prevent ' pumping ' of the mercury 

 surface at A. An asbestos cone, shown in section in 

 dotted lines in the diagram, surrounds the bulb and vertical 

 tube and a short asbestos tube protects the neck of the 

 bulb N. 



In starting the pump, the base of the instrument is tilted 

 until the end of the fall tube T is above the mercury surface 

 at A. The auxiliary pump is then worked, and the whole 

 apparatus exhausted as far as possible by this means. The 

 base is then re-levelled until the end of the fall tube is 

 sealed by the mercury at A to any desired amount, the 

 mercury boiled at B, ordinary precautions being taken 

 against sudden heating, and the final exhaustion effected 

 by the passage of the drops of mercury through the fall 

 tube. During this latter process the auxiliary pump need 

 only be worked intermittingly to remove the small traces 

 of gas which are carried over into the chamber P. The 

 current of water through the outer tube of the condenser 

 should be started before the inner tube gets heated by the 

 mercury vapour. 



If arrangements are made for the intermittent or con- 

 tinuous slow action of the auxiliary pump, when the flame 

 of the gas burner has been adjusted to evaporate the 

 mercury at a sufficiently rapid rate, the pump will work 

 indefinitely without further attention, provided the supplies 

 of gas to the burner and of water to the condenser are 

 unaltered. If air has to be admitted to the apparatus, the 

 base must first be tilted until the end of the fall tube is 

 clear of the mercury surface at A. 



The form of the condenser is not important; the one 

 shown in the drawing is that which was adopted after 

 some trials. In such a one the slope of the inner tube is 



