FILTRATION OF CULTURES. 



75 



filter consists practically of an ordinary tap screwed into the top 

 of a porcelain tube. Through the latter the fluid is forced and 

 passes into a chamber formed by a metal cylinder which sur- 

 rounds the porcelain tube. The fluid escapes by an aperture at 

 the bottom. Such a filter is very suitable for domestic use, or 

 for use in surgical operating-theatres. As considerable pressure 

 is necessary, it is evident it must be put on a pipe leading 

 directly from the main. Sometimes, when fluids to be filtered 

 are very albuminous, they are forced through a porcelain cylin- 

 der by compressed carbonic acid gas. In ordinary bacteriologi- 

 cal work, however, it is usually more convenient to suck the 

 fluid through the porcelain by exhausting the air in the recepta- 

 cle into which it is to flow. This is conveniently done by means 

 of a Geissler's water-exhaust pump (Fig. 38, g), which must be 

 fixed to a tap leading directly 

 from the main. The connec- 

 tion with the tap must be 

 effected by means of a piece 

 of thick-walled rubber tubing 

 as short as possible, wired on 

 to tap and pump, and firmly 

 lashed externally with many 

 turns of strong tape. Before 

 lashing with the tape the 

 tube may be strengthened by 

 fixing round it with rubber 

 solution strips of the rubbered 

 canvas used for mending 

 punctures in the outer case 

 of a bicycle tire. A ma- 

 nometer tube b and a re- 

 ceptacle c (the latter to 

 catch any back flow of water from the pump if the filter acci- 

 dentally breaks) are intercepted between the filter and the 

 pump. These are usually arranged on a board a, as in Fig. 38. 

 Between the tube / and the pump g, and between the tube d 

 and the filter, it is convenient to insert lengths of flexible lead 

 tubing connected up at each end with short, stout-walled rubber 

 tubing. 



Various modifications of the filter are used, (a) An apparatus 



Fig. 38. — Geissler's vacuum-pump arranged 

 with manometer for filtering cultures. (The 

 tap and pump are intentionally drawn to a larger 

 scale than the manometer board to show details.) 



