350 TEXT-BOOK OF BACTERIOLOGY. 



Petri passes the air to be examined for bacteria through a 

 small filter of previously-heated and, therefore, sterile sand. He 

 thus catches all the micro-organisms present. He then transfers 

 the sand into liquid nutrient gelatin, pours the latter into little 

 saucers, and observes the colonies as they develop. The sand has 

 a uniform grain of a diameter of to -J mm. It is brought (in 

 the form of small plugs, 3 cm. long and 1-J cm. thick, and supported 

 on both sides by a fine wire screen) into a glass tube, 8 to 9 cm. long, 

 in which two filters of the sizes just mentioned can be placed one 

 behind the other. Every thing being ready, the whole is once more 

 sterilized in the drying oven; one aperture of the glass is closed by 

 a rubber plug simply perforated, bearing a tight small glass tube, 

 which is now connected with a strong aspirating apparatus (an 

 oscillating air-pump being preferable), whose rotations can be 

 exactly controlled and indicate the quantity of air sucked in through 

 the sand (generally 10 litres in one to two minutes in Petri's experi- 

 ments). Each of the two little filters is then mixed with gelatin 

 and the work is continued. In properly-executed experiments the 

 filter adjacent to the pump should contain no germs at all, as they 

 should have been caught by the anterior filter. 



The results reached by all these observations generally agree. 



It has first been shown that the number of germs in the air is 

 by no means very large and that evidently the quantity of these 

 micro-organisms varies extremely as to place and time. 



It would lead us too far to discuss these conditions more in 

 detail. We will merely mention that the air of our dwelling-places 

 contains, on an average, three to five germs in a litre; that the 

 surrounding atmosphere is usually of the same composition; that 

 there are, on the whole, fewer micro-organisms in winter than in 

 summer; and, finally, that their proportion deviates considerably 

 from the mean, only under special circumstances, such as strong 

 motion, violent agitation, etc. The air of high regions is freer 

 from bacteria than that of low grounds, and the atmosphere on 

 the sea and the tops of mountains seems to contain no micro- 

 organisms. 



The kinds of germs developing into colonies in the culture-vessels 

 are likewise subject to great variations. Saccharomyces, penicillia, 

 and bacteria appear in confused masses, and among the last-named 

 micrococci are found in great varieties. Pathogenic species, 

 parasitic bacteria (except the Staphylococcus pyogenes aureus) 

 have not yet been surely detected in the air by direct investiga- 

 tion. 



