8o METHODS OF CULTIVATION OF BACTERIA. 



the bottle and not inserted in the latter till both are cold, 

 otherwise it will be so tightly held as to make removal very 

 difficult). In using such a bottle it is best to immerse it 

 in the water, and then remove the stopper with forceps. 

 Care must be taken not to touch the water-bed as the 

 vegetable matter covering it contains a large number of 

 organisms. Plates must be prepared from the samples as 

 soon as possible. 



Filtration of Cultures. For many purposes it is neces- 

 sary to filter all the organisms from fluids in which they 

 may have been growing. This is especially done in obtain- 

 ing the soluble toxic products of bacteria. The only filter 

 capable of keeping back such minute bodies as bacteria, is 

 that formed from a tube of unglazed porcelain as intro- 

 duced by Chamberland. There are several filters, differing 

 slightly in detail, all possessing this common principle. 

 Sometimes the fluid is forced through the porcelain tube. 

 In one form the 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 surrounds 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 cylinder by 

 compressed carbonic acid gas. In ordinary bacteriological 

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

 fluid through the porcelain by exhausting the air in the 

 receptacle into which it is to flow. This is conveniently 

 done by means of a Geissler's water- exhaust pump (Fig. 

 29, g), which must be fixed to a tap leading directly from 

 the main. The connection 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 



