no EXAMINATION OF AIR AND WATER 



8. Petri's method— 



In this method the air is aspirated through a glass tube of 

 about 9 cm. in length and from I-5-I-8 cm. internal bore containing 

 sterilised sand, which is kept in position by wads of iron wire 

 gauze. The sand is afterwards distributed into tubes of gelatine, 

 from which plate cultivations are made in the ordinary manner. 

 The presence of the opaque particles of sand in the gelatine 

 renders the accurate enumeration of the minute colonies a matter 

 of extreme difficulty, and is the principal objection to this method. 



9- Frankland's method— 



Frankland uses finely powdered cane sugar and glass wool 

 as a filtering medium. The form of tube employed is shown in 

 Fig. 34. A glass wool plug is inserted at c, and a second one 

 at b, consisting of glass wool and cane sugar in a finely powdered 

 condition ; this again is supported on each side by plugs of glass 

 wool. A cotton-wool plug is inserted at a and the tube is 



Fig. 34. — Frankland's air tube. 



sterilised in the hot air steriliser. When required for use the plug 

 a is removed and the tube connected up by a length of india- 

 rubber tubing to an aspirating apparatus. The plug c is then 

 withdrawn, and the aspirator set in action. After a sufficient 

 quantity of air has been passed through, the tube or tubes are 

 removed to the laboratory, a file mark is made across the centre of 

 ■each, the -tube itself is then broken in half and the plugs of glass 

 wool and powdered sugar are pushed, by means of a sterile needle, 

 into a flask containing 10 or 15 c.c. of liquefied gelatine. The 

 sugar dissolves, and the organisms contained in the glass wool 

 are distributed through the gelatine. A roll culture is made on 

 the walls of the flask, or Petri plates are made from the gelatine. 

 The difficulty in distinguishing the young colonies, owing to the 

 opalescence of the medium due to the presence of the glass wool, 

 is a great objection to this method. 



Bacteriological Examination of Water 

 Collection of samples. — Water from streams or wells should be 

 collected in glass bottles or flasks, closed with glass stoppers, 



