62 MICRO-ORGANISMS AND FERMENTATION. 



any germs which may have settled on the walls of the tube 

 may be washed down into the liquid. Then, by blowing 

 with greater force, the inner cotton- wool plug w is driven 

 down into the liquid, and its germs shaken off into the latter. 

 After sterilising the thin tube B in a flame, the point is nipped 

 off, and the liquid is now by blowing through Asp trans- 

 ferred, drop by drop, into a large number of flasks containing 

 sterilised broth. 



The main object then is, by means of preparatory experi- 

 ments, to obtain such a dilution of the air-infected water that 

 a considerable proportion of the small flasks (one-half for 

 example) remain sterile after inoculation ; or rseveal samples 

 of the water may be diluted to different degrees, and a series 

 of flasks inoculated from each dilution (" fractional cultiva- 

 tion "). If a large number of the flasks show no develop- 

 ment of organisms, there is a certain probability that in 

 each of the remaining flasks in which growths have developed, 

 only one germ has been sown. A simple calculation will then 

 show how many germs capable of development in the medium 

 employed were present in the volume of air aspirated through 

 the original flask. 



By these methods of investigation Miquel found that 

 similar volumes of air in the same locality contained at different 

 times a varying number of bacteria. Continued rain purifies 

 the air from bacteria to a marked extent, and their number 

 continually diminishes as long as the earth is moist ; but 

 when the ground dries, it gradually increases again. Thus 

 in the dry seasons of the year the number of bacteria is 

 usually the greatest, whilst the moulds, which thrive best 

 in moisture, and carry spore-bearing hyphae, which project 

 upwards, are most abundant during the wet seasons. The 

 purest air is found in the winter time ; the air of towns is less 

 pure than that of the country ; germ-free, or nearly germ- 

 free air is found at sea and on high mountains. In certain 

 places hospitals, for instance the air has been found to be 

 very rich in bacteria ; in one case even fifty times richer than 

 the air in the garden at Montsouris. 



An entirely different method for determining the organisms 

 contained in air is that employed in Koch's laboratory, and 



