CHAPTER LXVI. 

 THE BACTERIOLOGICAL EXAMINATION OF AIR. 



Introduction. 



1. Original methods, p. 862. 



2. Methods employed at the present day, p. 864. 



A. Methods based upon nitration, p. 865. 



B. Methods based upon bubbling the air through a suitable liquid, p. 866. 



THE bacteriological examination of air may be either quantitative or 

 qualitative, depending upon whether it is proposed to ascertain the number 

 of organisms present in a given volume of air or whether the object is to 

 determine to what species these organisms belong. Finally, the object of the 

 experiment may be to detect the presence of some given pathogenic organism. 

 Since the number of organisms in air is small the unit adopted is a cubic 

 metre : thus it is usual to say that the air of a room contains 500, 1000, or 

 3000 organisms per cubic metre as the case may be. 



For a long time it was considered sufficient to examine microscopically the dust 

 of the air collected by means of an aeroscope. The aeroscope most generally in use 

 in France is that of Pouchet. It consists of a glass cylinder closed at both ends : 

 inside, about the middle, an ordinary microscope slide is held by two supports and 

 on the upper surface of the slide a drop of glycerin is placed. The top of the glass 

 cylinder is perforated in the centre by a circular hole carrying a platinum funnel, 

 the tube of which passes into the cylinder above the centre of the slide. A tubulure 

 fixed to the lower part of the aeroscope is connected to an aspirator. When the 

 aspirator is working, the air passing through the funnel impinges upon the slide 

 and deposits its suspended dust which is retained there by the glycerin. When 

 a sufficient volume of air has been drawn through, the aspirator is turned off, 

 and the slide removed ; the dust is distributed by means of a sterile needle through 

 the glycerin which is then covered with a cover-glass and examined under the micro- 

 scope. In this way the larger particles of dust in the air may be studied spores 

 of fungi, moulds, pollen, starch grains, mineral particles, etc. but the method is 

 not sufficiently delicate for the detection of bacteria and their spores. At the present 

 time cultural methods are employed practically to the exclusion of all others. 



1. Original methods. 



I. Pasteur's method. Pasteur's method which is the oldest of all consists in the 

 use of long-necked flasks one-third filled with veal broth. The neck of each flask 

 is drawn out to a point, the flask and its contents are sterilized and the point sealed 

 in a small flame while the broth is still at the boiling point : in this way all the air 

 is driven out. It is now only necessary to carry the flask to the place where the air 

 is to be examined and to break off the fine point. The air with the particles in 

 suspension will rush into the flask, and as soon as it is full the point is sealed again, 



