io8 EXAMINATION OF AIR AND WATER 



covered with a thin surface of solidified nutrient gelatine. The 

 organisms in suspension in the air fall upon, and adhere to, the 

 surface of the gelatine during the passage of the air through the 

 tube, the subsequent colony growth being clearly visible through 

 its walls. The apparatus consists of a large tube of glass from 

 50-70 cm. in length, and from 3-5 cm. internal diameter, one end 

 of which is closed by a tightly drawn covering of india-rubber 

 sheeting pierced in the centre by a circular hole of i cm. in 

 diameter, this again being covered, until the moment of employment, 

 with a second covering of india-rubber securely tied on. The other 

 end of the tube is closed by an india-rubber stopper bored for a 

 length of glass tubing of small calibre, which in use is connected 

 up with the aspirating apparatus. This length of glass tubing 

 should project about i cm. into the interior of the larger tube and 

 be furnished at its outer orifice with a plug of cotton-wool. The 

 apparatus is then sterilised at 115° C. for half an hour. Before use 

 the outer india-rubber covering is removed and from 40-50 c.c. of 

 sterile nutrient gelatine is introduced into the tube by means of a 

 suitable delivery flask. The outer india-rubber covering is then 

 quickly replaced, and the tube itself rolled in the hand upon ice, or 

 under a cold water tap, until the gelatine becomes fixed in a thin 

 layer on its interior walls. When in use the tube is fixed horizon- 

 tally on a suitable stand, the length of glass tubing connected up with 

 an aspirating apparatus, the outer india-rubber covering at the 

 further end of the tube removed, and a current of air drawn through 

 the tube. The air should pass through slowly, and at a rate not 

 exceeding i litre per three minutes, in order to allow the organisms 

 in suspension to deposit themselves upon the surface of the 

 gelatine. After a sufficient quantity of air has been passed through, 

 the outer india-rubber covering is replaced, the tube detached from 

 the aspirator and the orifice of the exit tube again plugged with 

 cotton-wool. The tube is then placed under suitable condi- 

 tions for the development of the colonies, which should begin to 

 appear about the second or third day. The method is some- 

 what uncertain in its results, owing to the fact that a certain 

 proportion of the organisms are apt to pass through the tube 

 without being deposited, or are left adhering to the surface of the 

 india-rubber stopper. The gelatine too is apt to be deposited 

 unevenly upon the tube walls, and when too thin desiccates 

 quickly during incubation and becomes unsuitable for colony 

 growth. 



