the Mercurial Air-pump. 131 



Geissler's pump, which first became known to the general 

 public through Dr. Theodore Meyer's pamphlet on the stratified 

 electric light, published here [in Berlin] in 1858, does not differ 

 in principle from Swedenborg's, but its form is better adapted 

 to its special purpose. With the exception of the caoutchouc 

 tube, which is substituted for Swedenborg's leather joint, it is 

 made entirely of glass ; and in all that relates to the connexion 

 with the pump of the tubes to be exhausted, and the introduc- 

 tion and drying of different kinds of gas, its construction dis- 

 plays great neatness and skill. The contrivance for raising and 

 lowering the moveable tube, however, is but rough and liable to 

 be easily put out of order. This operation was at first per- 

 formed entirely by hand; but the use of a glass tube and a 

 charge of from 30 to 40 pounds of quicksilver being found too 

 troublesome and dangerous to be managed in this way, it was 

 afterwards done by help of a windlass. 



Quite recently Geissler of Berlin has constructed pumps of a 

 smaller pattern, containing only 15 pounds of mercury, in which 

 the two glass tubes are replaced by a long and strong tube of 

 caoutchouc, which connects the oval exhausting- vessel with a 

 glass globe of the same capacity. The operation of exhaustion 

 is performed by hand, by alternately placing the glass globe upon 

 the foot-board and upon a stand placed near the pump. This 

 arrangement is certainly simple, but must excite some appre- 

 hension as to how long the caoutchouc tube will last ; besides, 

 working the pump requires the exertion of a not inconsiderable 

 amount of force, and may easily expose the apparatus to danger. 



A mercurial pump, however constructed, is, on account of 

 its slow working, only adapted to exhaust vessels of small ca- 

 pacity; and although for these it may afford a better vacuum, 

 it will never displace the common air-pump from physical 

 cabinets. 



This conviction, together with the circumstance that now-a- 

 days every physicist is already provided with an air-pump, in- 

 duced me in the course of last summer to consider whether it 

 was not possible so to modify the mercurial pump as to make 

 it an appendage to the common pump, which might, according 

 to circumstances, be either used in conjunction with it or sepa- 

 rated from it. 



The execution of this idea presents no practical difficulty, 

 does not increase the price of the instrument, allows it to be con* 

 structed of any required size, and makes the operation of exhaus- 

 tion perfectly easy and free from danger. 



In order to be able to speak upon the subject from experi- 

 ence, I have had such an auxiliary instrument made as a trial, 

 and, as was to be foreseen, it has completely fulfilled my expec- 



