40 MICRO-ORGANISMS AND FERMENTATION. 



and a series of flasks inoculated from each dilution. When 

 a large number of the flasks do not show any development 

 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 ("fractional cultivation"). 



By these methods of investigation Miquel found that 

 similar volumes of air in the same locality contained at 

 different times a different number of bacteria. A prolonged 

 rain greatly purifies the air from bacteria, and their number 

 continually diminishes as long as the earth is moist ; but when 

 the ground dries, they again gradually increase. In the dry 

 seasons of the year the number of bacteria is thus usually 

 the greatest, whilst the mould-fungi, which thrive best in 

 moisture, and whose organs of reproduction project upwards, 

 are most abundant in the air during the w T et seasons. The 

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

 pure than that outside the towns ; germ-free, or nearly germ- 

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

 localities hospitals, for instance the air has been found to 

 be very rich in bacteria ; in one case even 50 times richer 

 than the air in the garden at Montsouris. 



An entirely different method for the examination of the 

 organisms contained in air is that employed in Koch's 

 laboratory, and more completely developed by Hesse. A 

 glass tube, about 1 meter long and 4 to 5 cm. wide, is closed 

 at one end with a perforated india-rubber membrane, over 

 which another non-perforated cap is bound. A little liquid 

 nutritive gelatine is then poured into the tube, after which 

 the other end of the tube is closed with an india-rubber 

 stopper, through which passes a glass tube plugged with 

 cotton-wool. The whole apparatus is then heated sufficiently 

 to render it sterile, after which the tube is placed in a 

 horizontal position, so that the gelatine sets in a layer in the 



