638 



BACTERIA IN WATER. 



nearly filled with water that it is not so simple a matter to obtain the 

 contents for our culture experiments without undue exposure to at- 

 mospheric germs. In practice small glass bottles with ground-glass 

 stoppers will be found most convenient, and, when properly steril- 

 ized, are unobjectionable. They should be filled at a little distance 

 below the surface, as there is often a deposit of dust upon the surface 



of standing water, and sometimes a 

 delicate film made up of aerobic bac- 

 teria. When water is to be obtained 

 from a pump or a hydrant it should 

 be allowedrto flow for some time before 

 the collection is made. To collect 

 water at various depths the apparatus 

 shown in Fig. 197 is recommended by 

 Lepsius. An iron frame supports an 

 inverted flask, A, filled with sterilized 

 mercury and containing about three 

 hundred cubic centimetres. The flask 

 B is intended to receive the mercury 

 when, at the desired depth, it is al- 

 lowed to flow through the capillary 

 tube b. This is sealed at the extremity 

 and bent as shown in the figure. By 

 pulling upon the cord c this tube is 

 broken, and as the mercury flows from 

 the flask this is filled with water 

 through the tube a. The extremity 

 of the broken tube b is closed by the 

 mercury in the flask B when A is full 

 of water, and the apparatus can be 

 brought to the surface with only such 

 water as was collected at the depth 

 from which a sample was desired. 



The bacteriological analysis is 

 made by adding a definite quantity 

 of the water under investigation to 

 liquefied gelatin or agar-gelatin, and 

 making a plate or Esmarch roll tube, which is put aside for the devel- 

 opment of colonies. Miquel and others have preferred to use liquid 

 cultures and the method of fractional cultivation described in the 

 previous section. The use of a solid culture medium has, however, 

 such obvious advantages that we do not consider it necessary to do 

 more than refer to the other method as one which, when applied 

 with skill and patience, may give sufficiently accurate results. 



FIG. 



