CHAPTER IX 
BACTERIAL EXAMINATION OF AIR 
Tue bacteriological examination of air has not been of much sanitary 
significance. Many investigations have been carried out and different 
apparatuses devised to remove the bacteria from a definite quantity of 
air. A review of these methods has been given by Besson and those 
wishing a rather complete historical survey should consult that treatise. 
Ruehle (1915) has devised the most satisfactory technique and probably 
his method represents the best which has been devised up to the present. 
Much of the matter presented in this chapter has been taken from his 
publication. This method involves the use of the aeroscope, which is 
regarded as an apparatus for gathering bacteria from the air. The 
other accessories to the procedure are not included in this procedure. 
Rettger (1910) made one of the first important advances in the 
bacterial examination of air. He used 5 c.c. of physiological salt solu- 
tion as the filtering medium in a special aeroscope described as follows: 
The entire apparatus consists of a glass tube with a small round bulb at the 
end. The bulb has eight or ten small perforations which serve the purpose of 
allowing the air to pass through at a rapid rate and divide the gas to such an 
extent that every particle of it is brought into close contact with the filtering 
fluid. This glass tube or aeroscope is fitted into a small, thick-walled test tube 
by means of a rubber stopper, which also bears, besides the aeroscope, a short 
glass tube bent at right angles. The upper end of the aeroscope is at an angle of 
about 45°, in order to prevent bacteria and particles of dust from falling into 
the open end of the tube, and still permit of the tube being drawn through the 
stopper without difficulty. 
The standard aeroscope has been described by Ruehle as follows: 
A 10 mm. layer of sand which has been passed through a 100-mesh sieve 
and has been retained by a 200-mesh sieve is supported within a cylindrical glass 
tube 70 mm. in length and 15 mm. in diameter upon a layer of bolting cloth 
folded over the end of a rubber stopper. Through a perforation in the stopper, 
there passes a tube 6 mm. in diameter and 40 mm. in length. This tube is 
attached to the aspirator bottle. ‘The upper end of the cylindrical tube is closed 
by a perforated rubber stopper through which is passed a glass tube 40 mm. in 
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