252 The Bacteriology of the Air 



from 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. Petri points out 

 in relation to his method that the filter catches a relatively 

 greater number of bacteria in proportion to molds than the 

 Hesse apparatus, which depends upon sedimentation. Stern- 

 berg points out that the chief objection to the method is the 

 presence of the sand, which interferes with the recognition 

 and counting of the colonies in the gelatin. 



Sedgwick 's Method. Sedgwick and Miquel have recom- 

 mended the use of a soluble material granulated or pulver- 

 ized sugar instead of the sand. The apparatus used for 

 the sugar experiments differs a little from the original of 

 Petri, though the principle is the same, and can be modified 

 to suit the experimenter. 



A particularly useful form of apparatus (Fig. 71), sug- 

 gested by Sedgwick and Tucker, has an expansion above the 

 filter, so that as soon as the sugar is dissolved in the melted 

 gelatin it can be rolled out into a film like that of an Esmarch 

 tube. This cylindric expansion is divided into squares 

 which make the counting of the colonies very easy. 



Roughly, the number of germs in the atmosphere may be 

 estimated at from 100 to 1000 per cubic meter. 



The bacteriologic examination of air is of very little im- 

 portance because of the numerous errors that must be met. 

 Thus, when the air of a room is quiescent it may contain 

 very few bacteria ; let some one walk across the floor so that 

 dust rises, and the number of bacteria becomes considerably 

 increased; if the room be swept, the increase is enormous. 

 From these and similar contingencies it becomes very diffi- 

 cult to know just when and how the air is to be examined, 

 and the value of the results is correspondingly lessened. 



The most sensible studies of the air aim rather at the dis- 

 covery of some definite organism or organisms than at the 

 determination of the total number per cubic meter. 



