BACTERIOLOGIC EXAMINATION OF AIR 179 



4. Place the Erlenmeyer flask with the bouillon in a glass, metal, 

 or other suitable vessel, and pack it around with cracked ice. Place 

 the Erlenmeyer flask on a stand or box so that the bent glass tube 

 in the perforated stopper can be conveniently connected by a rubber 

 tube with the aspirating bottle. 



5. Remove the spring clip from the rubber tube and allow the 

 water to run from the aspirating bottle at the rate of 1 c.c. a second, 

 as previously arranged. 



6. From time to time replace the ice in order to keep the bouillon 

 near C. in order to prevent multiplication of the bacteria in it. 



6. When the 10 liters of water have run out of the aspirating 

 bottle, a corresponding quantity of air has been aspirated through the 

 50 c.c. of bouillon in the Erlenmeyer flask. The bouillon now con- 

 tains all the bacteria that were originally in the 10 liters of aspirated 

 air. Then disconnect the Erlenmeyer flask with the 50 c.c. of bouillon 

 and shake the contents well. 



7. In the meantime a number of gelatin or agar tubes have been 

 liquefied and cooled down to near the point of solidification of the 

 media. Then with sterile graduated pipette add some of the bouillon 

 to the fluid culture media in these tubes. To the first add 0.5 c.c., 

 to the next 0.3 c.c., and to a third 0.2 c.c. The contents of the tubes 

 are next poured into two sets of sterile Petri dishes, one of which is 

 kept at room and the other at incubator temperature. After a few 

 days the colonies are counted and the average per cubic centimeter 

 of bouillon is calculated. This average multiplied by fifty indicates the 

 number of bacteria contained in the 50 c.c. of bouillon in the Erlen- 

 meyer flask and represents the bacteria present in the 10 liters of 

 air which have been aspirated through the apparatus. The number 

 of bacteria present in air is usually stated in terms of cubic meters, 

 and since a cubic meter is equal to 1000 liters, the last figure must 

 be multiplied by 100 to obtain the number of bacteria in one cubic 

 meter. 



METHODS OF FRANKLAND AND PETRI. Frankland and Petri have, 

 independent of each other, devised a method in which the air is first 

 aspirated into a filter of sterile quartz sand. The grains of sand have 

 an average diameter of ^ to -J mm., and are contained in a piece of 

 glass tubing 6 to 10 cm. long and about 2 cm. wide. The sand is 

 held in the centre of the tube, and divided into two equal portions by 

 fine wire gauze. One end of the tube is closed by a cotton plug and 

 the other, after thorough dry sterilization, by a perforated rubber 

 stopper containing a small glass tube. When the device is to be 

 used the latter is connected with an air pump. The tube containing 

 the sand filter is held upright during the process of aspiration which 

 is continued at the rate of 10 liters per minute for from ten to twenty 

 minutes. The upper and lower portions of sand are then separately 

 mixed with liquefied gelatin or agar, well shaken, and poured into 

 Petri dishes. The lower portion of sand should be found sterile, the 



