->r,4 APPENDIX. 



than one large one ; and when the fluid ceases to pass through 



of filters, to' pour it into fresh ones. 



ition // Iliinten's P ////,'. Filtration maybe much accelerated 



l,v lilterin- the liquid into :i partial vacuum. This is done by fixing the 

 funnel air-tight in one neck of si Woull'e's bottle, and exhausting the air 



throu-h the' other by an ordinary exhausting syringe. It can, however, 

 l,r more conveniently effected by means of a Bunsen'fl water air-pump 

 885). 



Tin- principle of this instrument is the same as that of Sprengel's pump, 

 with this ditl'erenee. that water is substituted for mercury. It consists of 

 a wide air-tiirht tube, through which water descends in a constant stream 



pth Which (if it is desired to produce a complete vacuum) must 

 not hi- less than thirty-two feet. Into the axis of this tube, close to its 

 upper end. a second tube of much smaller bore projects, the open end 

 of which looks down\\ anls. /. e., in the direction of the stream. Through 

 this tuli.', if it is opi-n. air is constantly drawn ; any closed cavity with 

 which it is in air-tight communication is rapidly exhausted. It may 

 thus be used eitlier as an aspirator or as an air-pump. If, however, the 

 .f the column of water is less than thirty-two feet, its exhausting 

 poueris limited to the production of a diminished pressure, which is 

 i-xpre ed by the ditl'erenee bet ween the height of the column and thirty- 

 two feet. The usual \\av of employing it in filtration is to attach the 



ion tube u to a piece of bent glass tubing, which passes through 

 an India-rubber stopper in one neck of a Woulfe's bottle, in the other 



(f which a funnel is fixed in a similar manner. The air inside the 



bottle being exhausted by the air-pump, the fluid is forced rapidly 



through the filter by the pressure of the external atmospheie. 1 find it 



:iient to UM- a strong bell jar. with a tubular opening at the 



Into this opening an India-rubber stopper, which is perforated for 



the funnel and exhausting tube, is fitted. The beaker in which the fil- 



ived is placed on a ground-glass plate. The ground 



if the bell jar having been .-.mcared with resin ointment, it is set 

 on the plate iii such a position that the funnel is exactly over the beaker. 

 The tluid is then poured into the filter, and the air exhausted from the 

 bell jar. The pressure of the air would force the liquid through the 



did tear it away unless it wire supported in some way. This is 

 done by taking a semicircular piece of platinum foil of suitable si/e. A 

 snip ha\ing been made at the centre of the straight edge, and at right 

 angles to it. the bit of toil is heated in the blowpipe {lame, and allowed 



!. Il can then lie smoothed out. bent at the snip, and the edges 

 brought together so a-- to overlap each other slightly. The hollow cone 

 thus formed is next placed in an iron mould with a conical cavity, and 



1 firmly in with a conical plug. The funnel used must be chosen 

 with sides sloping at the proper anirlc. and the tube must not be too wide, 

 at the junction with the cone. The platinum toil is placed in the bottom 

 of the tunnel, and prosed with the linger. > as to lit the funnel smoothly. 

 In-tead of platinum foil, fine wire ^au/.e or parchment paper is som'e- 



used. The filter is then folded and placed Aith its apex resting in 

 the platinum, moistened with water, and pressed gently a gainst the sides 

 of the funnel so as to make it lit tightly to it. ami prevent air from L r et- 

 tin^ down into the receiver between them. Milk, albuminous solutions, 



'\eriin can be tillered much more readily through j>oroiis eartlien- 



iian thro\iu!i ]>apcr. For this purpose the top of a porou> cell, 

 sneli ;,-, i^ US( .,1 lor u-alvanic batteries, is closed by an India-rubber cap 

 with t\\ o peninirs. < )ne ot'these is connected by a short glass tube and 



: India-rubber tubing with the pump. Thnumh the other a <;lass 



- nearly to the bottom of the cylinder, and is closed at its 



piece of India-rubber tubing and a strong clip. This 



