616 BACTERIA IN THE AIR. 



mixture of glycerin and glucose has been placed, is adjusted near the 

 opening of the funnel, at a distance of about three millimetres, so 

 that the air escaping through the small orifice is projected against it. 

 By this arrangement a considerable number of the microorganisms 

 present in the air, as well as suspended particles of all kinds, are ar- 

 rested upon the surface of the slide and can be examined under the 

 microscope or studied by bacteriological methods. But an aeroscope 

 of this kind gives no precise information as to the number of living 

 germs contained in a definite quantity of air. The microscopical ex- 

 amination also fails to differentiate the bacteria from particles of 

 various kinds which resemble them in shape, and the microorgan- 

 isms seen are for the most part spores of various fungi mingled with 

 pollen grains, vegetable fibres, plant hairs, starch granules, and 

 amorphous granular material. 



Another method, which has been employed by Cohn, Pasteur, 

 Miquel, and others, consists in the aspiration of a definite quantity of 

 air through a culture liquid, which is then placed in an incubating 

 oven for the development of microorganisms washed out of the air 

 which has been passed through it. This method shows that bacteria 

 of different species are present, but gives no information as to their 

 relative number, and requires further researches by the plate method 

 to determine the characters of the several species in pure cultures. 



A far simpler method consists in the exposure of a solid culture 

 medium, which has been carefully sterilized and allowed to cool on a 

 glass plate or in a Petri's dish, for a short time in the air to be ex- 

 amined. Bacteria and mould fungi deposited from the air adhere to 

 the surface of the moist culture medium, and form colonies when the 

 plate, enclosed in a covered glass dish, is placed in the incubating oven. 

 The number of these colonies which develop after exposure in the 

 air for a given time enables us to estimate in a rough way the num- 

 ber of microorganisms present in the air of the locality where the 

 exposure was made ; and the variety of species is determined by ex- 

 amining the separate colonies, each of which is, as a rule, developed 

 from a single germ. By exposing a number of plates at different 

 times this method enables us to determine what species are most 

 abundant in a given locality and the comparative number in dif- 

 ferent localities, as determined by counting the colonies after ex- 

 posure for a definite time e.g., ten minutes. Of course we will only 

 obtain evidence of the presence of such aerobic bacteria as will 

 grow in our culture medium. The anaerobic bacteria may be studied 

 by placing plates exposed in a similar way in an atmosphere of hydro- 

 gen. Bacteria which grow slowly and only under special conditions, 

 like the tubercle bacillus, would be likely to escape observation, as 

 the mould fungi and common saprophytes would take complete pos- 



