454 



BACTEBIOI^GY. 



where the air is to be examined; the outer cap is removed and the watei: 

 in the aspirating bottle is then allowed to escape. Any desired voIt 

 ume of air can thus be drawn through the tube. The bacteria pres- 

 ent in the air, coming into contact with the gelatin, become fixed and 

 after the tube is set aside they develop. The colonies can then be 

 counted, and, inasmuch as the volume of air drawn through the apr 

 paratus is known, the average number of germs per liter of air can 

 be ascertained. 



A more exact method, requiring however very expen- 

 sive apparatus, is that of Petri. In this method the air is 

 drawn through a sterile sand filter 

 Fig. 66 d, p. 469; The suspended 

 particles, dust and organisms,- are 

 held back by the well-packed, fine 

 grained sand. The contents of the 

 filter are then distributed into a num- 

 ber of Petri dishes, gelatin is added 

 and after thorough mixing it is al- 

 lowed to solidify. The colonies that 

 now develop can be examined and 

 counted with the same^ ease as in 

 water analysis. In this method it is 

 possible for a large number of bac- 

 teria to adhere to a single grain of 

 sand and when development takes 

 place the result will be but one 

 colony. The objection can be over- 

 A ^ come, in part, by substituting a sol- 



natiM of air."*" -s°ed^™k- ublfe compound for the sand. Thus, 



Tucker's; 5— Straus-Wurtz's. , , , . , , . n j.j. 



powdered sodium sulphate, or better, 

 cane-sugar may be employed. Pilfers containing sugar 

 have been employed by Miquel, and by Sedgwick: and 

 Tucker (Pig. 60 a). On the addition of gelatin, the s'ugar 

 dissolves leaving the germs present in a fine state of 

 division. The nuinber of colonies, thus obtained, repre: 

 sents quite closely the number of cells originally present 

 in the air. 



