Mr. J. B. Hannay's Examination of Vacua. 231 



After some preliminary trials the apparatus shown in 

 fig. II. was constructed. The bulb A, which is connected 

 with B by a ground joint, has its lower extremity ground to 

 fit the upper part of the bulb C. D is a tube leading to the 

 mercury Sprengel. E are the bulbs to be evacuated. F is a 

 narrow tube which allows the metal to run very slowly into 

 the pump G-. The stopcocks and ground joints were lubri- 

 cated with Mr. Crookes's invention, burnt india-rubber, which 

 is the only substance of use at high temperatures. I think it 

 acts better at high than at low temperatures, because it soon 

 hardens up on the outside to a hard varnish-like body which 

 is quite impervious to gases. The india-rubber must be the 

 pure variety, such as is used for toy balloons. 



The modus operandi is as follows : — The apparatus being 

 fixed inside the bath with the joints all made, the stopcocks a 

 and d are closed and b and c left open. The ordinary 

 mercury-pump is then set in motion till a good vacuum is 

 obtained. During the evacuation the stopcock d is slightly 

 opened, the point being dipped in fusible metal, and the metal 

 allowed to rise till it is just inside d, and the cock then 

 closed. 



The fusible metal is poured into C through a, care being 

 taken to open a only very slightly at first. It was found 

 that the fusible metal adheres to the glass, and that no air is 

 admitted with the metal, as would be the case with mercury. 

 When all the metal is in C, the connexion with the Sprengel 

 pump at D is sealed off. The metal is now running very slowly 

 down through F, and acting as a Sprengel pump through Gr, 

 evacuating the bulbs E. The stopcock b is closed, so that the 

 metal collects in B. When B is nearly full the metal is 

 transferred to the top by the following method: — b and c are 

 opened, transferring the metal from B to A, and then closed, 

 and A disconnected from B; it is then raised, and adapted 

 to C, the upper part of which beyond , the stopcock a is 

 filled with fusible metal. The tube below d also being full, 

 no air is allowed to get in; d and a are then opened, when the 

 metal runs into C. A is now returned to the bottom for a 

 new charge. There is great difficulty in handling the bulbs, 

 owing to the heat, but with flannel and proper clamps the 

 work can be got through. During the evacuation the bulbs 

 E are heated nearly to the softening-point, so as to free them 

 from air ; and after an hour or so, B is allowed to fill up full 

 and the metal to pass up into E, sweeping any residual gas 

 into the upper tube H, where it is imprisoned by freezing 

 the metal by a blast of cold air. B is then emptied, 

 and the exhaustion again proceeded with, E again heated 

 U2 



