BACTERIOLOGIC EXAMINATION OF AIR. 231 



L 



the upper filter is then carefully mixed with sterile 

 melted gelatin and poured into sterile Petri dishes, where 

 the colonies develop and can be counted. Sternberg re- 

 marks that the chief objection to the method is the pres- 

 ence in the gelatin of the slightly opaque sand, which 

 interferes with the recognition and count- 

 ing of the colonies. This objection has, (x^% 

 however, been removed by Sedgwick and 

 Miquel, who use a soluble material — granu- 

 lated or pulverized sugar — instead of the 

 sand. The apparatus used for the sugar- 

 experiments differs a little from the original 

 of Petri, but the principle is the same, and 

 can be modified to suit the experimenter. 

 Petri points out in relation to his method 

 that the filter catches a relatively greater 

 number of bacteria in proportion to moulds 

 than the Hesse apparatus, which depends 

 upon sedimentation. 



A particularly useful form of apparatus 

 is a granulated sugar-filter suggested by 

 Sedgwick and Tucker, which has an ex- 

 pansion above the filter, so that as soon as 

 the sugar is dissolved in the melted gela- 

 tin it can be rolled out into a lining like 

 that of an Esmarch tube. This cylindrical 

 expansion is divided into squares which 

 make the counting of the colonies very easy 

 (Fig. 46). 



The number of germs in the atmosphere Fig. 46.— Sedg- 

 will naturally be very variable. Roughly, ™ e S f J xp a ™ex- 

 the number may be estimated at from 100 amination. 

 to 1000 per cubic meter. 



In reality, the bacteriologic examination of air is 

 of very little value, as so many possibilities of error 

 may occur. Thus, when the air of a room is quiescent 

 there may be very few bacteria in it ; let some one walk 

 across the floor and dust at once rises, and the number 



