398 



Mr. C. H. Gimingham on a new 



[Dec. 7, 



tube in which the residual gas is condensed, and r another graduated 

 tube, where the pressure is taken. 



The supply of mercury for condensing the gas is taken from the 

 reservoir A of the pump, passing through the pinch-cock JN" up the 

 tube p *. 



The mercury joints used to connect the different parts of this pump 

 are small blown funnels carefully stoppered, shown in section (fig. 2) : 

 a, represents the funnel ; h, the stopper ; c, mercury ; d, sulphuric acid. 



The stopper is ground by hand with fine emery and water into the 

 neck of the funnel, using " rotten-stone " to finish Math. Stoppers made 

 carefully in this way need no grease, as the mercury and sulphuric acid 

 will not pass between surfaces fitting so closely. Sulphuric acid is used 

 to ensure perfect contact between the glass and mercury. 



Before going further, I will describe the vacuum-tap, which constitutes 

 one of the principal improvements in this pump, and will, I think, prove 

 very valuable to many who may conduct research at high exhaustions. 



It consists of three parts (A, B, C, fig. 3) : A is an ordinary stopper 

 ground to fit perfectly in B ; the lower end of the funnel B is a closed 

 stopper fitting very accurately in C. 



A neat hole (cZ), drilled through the centre of the stoppered or 

 ground part of B, meets a rather deep groove {e) cut in the funnel C 

 rather more than halfway up the ground part. When these three are 

 put together (lubricating the stoppers with a very little grease or burnt 

 india-rubber), the funnel B turns independently of A and C, so that 

 the latter parts can be fixed in. any way necessary. 



The tap is closed with the funnel B in any position, except that in 

 which the aperture d is opposite the groove e, when of course A is 

 in connexion with C and any apparatus attached. The stoppers are 

 proof against leakage on account of their working under mercury and 

 sulphuric acid placed in the funnels. When the tap is turned off, in 

 order to make it absolutely proof against leakage, supposing the tube A 

 or any thing connected with it has to be cut or opened after exhaustion, 

 before doing so the stopper A is slightly lifted to let a drop of mercury 

 fall from the funnel and cover up the little aperture d. The tap is now 

 a perfect mercury joint, allowing any apparatus to be taken off or fastened 

 to A without the slightest fear of deteriorating the vacuum below C. 



To again connect the tap with the pump (and therefore any apparatus 

 to which it is blown) the stopper A is removed, and the mercury 

 covering the small aperture d taken out with a fine pipette ; a small 

 globule of mercury will still remain in the little aperture, which, if desired, 

 may be taken out with an amalgamated copper wire ; otherwise, when the 

 tap is turned on, it will fall on the side of least pressure, viz. the side 

 best exhausted. 



* For detailed description of tkis apparatus see McLeod's paper in Phil. Mag. for 

 August 1874, p. 110. 



