BY DR. BURDOX-SANDERSON. 207 



the cavity to be exhausted. The pump is worked in the same 

 manner as that just described. In order, if necessary, to dry 

 the vacuum, a Pfliiger's drying apparatus is interposed between 

 the pump and the recipient. This may be described as a U 

 tube, the bend of which is dilated into a bulb (c). It is so 

 constructed that the fragments of pumice or the glass balls 

 moistened with sulphuric acid which are used for drying can 

 be readily introduced into either limb. The tube leading from 

 the dessicator to the pump communicates with a vacuum gauge 

 (m). The advantage which this instrument possesses consists 

 in the relatively large size of the bulb, the perfection of the 

 workmanship (particularly of the stopcocks) and the arrange- 

 ment whereby the vacuum obtained is dry. 



28. Frankland-Sprengel Pump. Sprengel's pump as 

 modified by Frankland, consists essentially of a vertical glass 

 tube (o Fig. 199) about four feet long, with thick walls and nar- 

 row bore, the lower end of which is bent up in such a way that, 

 if filled with mercuiy, and closed at the top, it would constitute 

 a barometer. At its upper end, however, it is not closed, but 

 is continuous by a bend with the second vertical tube (G) or 

 ascending limb of the Sprengel (the supply tube), which is of 

 wider bore, and runs parallel to the first. At the top, or con- 

 vexity of the bend, a third tube, about four inches in length 

 (the exhaustion tube), is sealed on, by which the barometer 

 tube or descending limb communicates with the cavity to be 

 exhausted. The ascending limb communicates by a flexible 

 tube, strengthened by a covering of strong canvass and 

 guarded by a screw clip, with the descending limb of another 

 bent tube (c) of similar construction to the first ; the only dif- 

 ference between it and the one just described being that it com- 

 municates at the bend, not with any cavity, but merely with a 

 bulb (D) closed at E by mercury. Its other limb finally com- 

 municates by a second flexible tube with a reservoir of mer- 

 cury (B), the arrangement of which will be best understood from 

 the figure. It consists of two glass funnels, each having long 

 stems, the relative sizes of which are such that the one can be 

 contained within the other. To work the pump, the exhaust- 

 ing tube of the first bent tube must be connected with the cavity 

 to be exhausted by means of a junction of vulcanized caout- 

 chouc, guarded by a chamber filled with glycerin. Mercury is 

 then poured into the inner funnel (the tube leading to the first 

 bend having been previously closed) until it rises in the space 

 between it and the outer to the same level. This done, the 

 clip is opened, and a stream of mercury is allowed to flow over 

 the two bends in succession, great care being taken that the 

 stream is not so abundant as to cause the mercury to ascend 

 in the exhausting tube above the level of the bend. The flow 

 must then be gradually diminished with the aid of the clip, 



