130 LABORATORY EXERCISES IN BACTERIOLOGY. 



life by infection of these when accidentally or carelessly exposed to the air. (In the 

 last connection, however, and in the transmission of disease to individuals the role 

 of flies and other insects as conveyers of the infections must be kept in mind.) Both 

 vegetative and spore forms of bacteria are included in the atmospheric flora. For 

 the most part they are attached to particles of dust or of moisture in the air, and for 

 this reason are more numerous in the lower than in the upper strata of the atmosphere, 

 and in dusty or moist air rather than in a clean and dry air. It is to be presumed 

 that unless exceptionally they do not develop in the air, but are conveyed by currents 

 from some point of origin ; for which reason they are more common in the atmosphere 

 of populous districts, where there is abundant opportunity for the development of 

 parasitic and saprophytic germs, than in that of dry, barren, unpopulated areas of 

 land or in the air over the ocean far from the shore ; and for the same reason, in the 

 atmosphere of warm, moist localities and in the summer, rather than in cold, dry climates. 

 So, too, one may expect to meet in the air of any selected locality more bacteria when 

 there are strong currents than when it is still. Cornet has estimated the existence 

 of over three hundred bacteria in a cubic meter of the air passing over the house-tops 

 of the city of Paris; and thousands may be demonstrated in the same volume of air 

 from the gutters and dirty cellars of cities. 



For collecting and cultivating these atmospheric organisms a variety of devices 



& 



FIG. 40. 

 A. Sedgwick-Turner aero-bioscope. B. Glass tube of Hesse's apparatus. 



have been suggested, of which the following two may be described as illustrative of 

 the principles of most : 



Sedgwick-Turner Aero-bioscope. This appliance (Fig. 40 A), as furnished in the 

 market, consists of a glass tube of the shape shown in the diagram, and is about thirty- 

 five centimeters in total length. The wide part of the tube is fifteen centimeters in 

 length and four and a half centimeters in diameter, narrowed at its wide end to a 

 neck two and a half centimeters in diameter; and at the opposite end continued as 

 a narrow straight tube fifteen centimeters long and one-half centimeter in diameter. 

 The wide part of the tube has squares marked upon its surface to facilitate counting 

 the colonies of bacteria scattered over the inner surface when the apparatus is in use. 

 A small roll of wire gauze is placed in the narrow tube just within the cotton stopper, 

 extending to a mark cut in the glass one-third of the length of this narrow part from 

 its free end. Both ends of the appliance are plugged with cotton and the whole steril- 

 ized in the oven in the usual manner. Some finely granulated sugar is now introduced 

 into the wide end of the tube, sufficient to evenly fill the narrow tube above the gauze 

 on which the sugar rests. This sugar is intended as a filter upon which the bacteria 

 of the air will lodge and be retained as air is drawn through the apparatus from the 



