APPARATUS 



93 



consequently close the tube and form a safeguard against the 

 air, which might enter from below if the equilibrium should be 

 disturbed. The upper part of C d branches off at x into a 

 lateral tube to which the receiver R is 

 affixed. As soon as the stop-cock at C is 

 opened and the mercury allowed to run 

 down, the exhaustion begins, and the whole 

 length of the tube from x to d is seen to be 

 filled with cylinders of mercury and air having 

 a downward motion. Air and mercury escape 

 through the spout of the bulb B, which is 

 above the basin H, where the mercury is 

 collected. This has to be poured back from 

 time to time into the funnel A, to pass 

 through the tube again and again until the 

 exhaustion is completed. As the exhaustion 

 is progressing it will be noticed that the en- 

 closed air between the mercury cylinders 

 becomes less and less, until the lower part of 

 Cd presents the aspect of a continuous 

 column of mercury about 30 inches high. 

 Towards this stage of the operation a con- 

 siderable noise begins to be heard similar to 

 that of a shaken water-hammer, and com- 

 mon to all liquids shaken in a vacuum. 

 The operation may be considered completed Fl( , 4i 



when the column of mercury does not enclose SPRENGEL'S MERCURY 

 any air, and when a drop of mercury falls PUMP. 



upon the top of this column without enclosing the slightest 

 air bubble. The height of this column now corresponds exactly 

 with the height of the column of mercury in the barometer ; or 

 what is the same, it represents a barometer whose Torricellian 

 vacuum is the receiver R." 



As a matter of fact the pump in this very simple form does not 

 give a perfect vacuum, for air adheres to the surface of the glass 

 funnel and tube, and india-rubber joints do not exclude the 

 entrance of minute quantities of air. The mercury requires to 

 be admitted to the fall tube with greater precaution, and the 

 whole must be constructed of glass without rubber connections. 

 Figure 35 (p. 65) shows a pump of this kind as mounted 



H 



