250 Mr. T. E. Thorpe on an Improved form ofFilter-Pump. 



The edges of the slit were thus left sharp -, and on applying any- 

 outward pressure to the tube they readily adhered, making a 

 perfectly air-tight conjunction. The valve was then pushed 

 within the tube B, which was narrowed at the end so as to retain 

 the caoutchouc tube perfectly air-tight. The other end of the 

 caoutchouc tube D D was connected with the vessel to be eva- 

 cuated. On allowing the water from the main to flow through 

 the vertical tube, the caoutchouc tube commences to pulsate 

 rapidly as it falls over the upper edge of the tube A, and periodi- 

 cally closes the opening. The Bunsen valve in consequence 

 intermittently opens and shuts^ and rapid suction is set up ; and 

 it is thus easy to obtain a vacuum equivalent to Ov metre of 

 mercury. The working of the apparatus is obviously akin to 

 that of the hydraiilic ram; so easily and efficaciously does it 

 exhaust, that it vvdll doubtless take the place of the Bunsen filter- 

 pump. It has the great advantage of portability over the older 

 form, since it may be so constructed that it can be transported to 

 any position in the laboratory : it obviates the necessity of a fall 

 of upwards of 30 feet, and therefore requires no alteration in the 

 existing arrangements of pipes and fittings ; and^ lastly, its cost 

 need not exceed a fevv^ shillings. 



- There are a few disadvantages connected with the use of the 

 caoutchouc valve above described. Owing to the gradual dimi- 

 nution of its elasticity by long-continued working, its efficacy 

 diminishes after a time; it not only then fails to bring about 

 rapid exhaustion, but so soon as the conjunction of its edges 

 ceases to be perfect, it allows the water to flow back into the 

 caoutchouc tube. To obviate these inconveniences another form 

 of valve was devised. A hollow metal cone shaped like a funnel 

 is soldered air-tight into the end of the side tube B (fig. 2). This 

 cone is pierced near its apex with a number of holes, and into it 

 is fitted a piece of unvulcanized sheet caoutchouc shaped like a 

 filter. This is retained in its place by a small screv/ passing through 

 the sheet caoutchouc and into the apex of the cone. By its 

 elasticity the india-rubber sheet presses firmly against the sides 

 of the cone and effectually prevents the entrance of air or water 

 from the tube A ; but the slightest pressure from within B is suf- 

 ficient to disturb the adhesion, and to allow of the ready trans- 

 mission of air through the holes in the cone. This valve is of a 

 more durable and permanent character than the original form, 

 and permits of a more rapid exhaustion. In the modified form 

 of the instrument a manometer M is fixed to B : this allows the 

 degree of exhaustion to be immediately ascertained from the 

 position of the mercury along the graduated scale. The rapidity 

 of the pulsations in the caoutchouc tube W ^^Y may be regulated 

 by the moveable arm T, which by means of a screw can be clamped 



