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BACTERIOLOGICAL EXAMINATION OF AIR. 



To determine the number and kind of bacteria present 

 in the air is a problem of considerable importance, and can 

 be accomplished quite satisfactorily with Hesse's appara- 

 tus. The large, wide tube is sterilized, nutrient gelatin 

 introduced, and a large Esmarch roll-tube then made. The 

 tube thus prepared is connected with an aspirating bottle 

 of known volume. In this way a definite volume of air 

 can be drawn through the apparatus. The bacteria pres- 

 ent in the air are deposited on the moist gelatin walls of 

 the tube, and subsequently develop, forming colonies- 

 These are counted and the number of colonies per liter of 

 air is thus ascertained. The kind of bacteria present can 

 be determined in the usual way. 



The method of Petri, though somewhat more compli- 

 cated, requires less time and gives excellent results. The 

 air is filtered by means of an aspirator or air pump through 

 a tube filled with sterilized sand. The sand, which then 

 contains the bacteria originally in the air, is transferred to 

 a Petri dish containing gelatin, thoroughly mixed, and set 

 aside to develop. 



The number of bacteria present in the open air is very 

 small and rarely exceeds 3-4 per liter. Usually the num- 

 ber is much less than this. Spores of moulds are more 

 abundant in the air than are bacteria. Air of mid- ocean 

 and of high altitudes is practically free of microorganisms. 



