40 MICRO-ORGANISMS AND FERMENTATION. 



side is another glass tube, which is connected by rubber, k> 

 with the tube B, which is drawn out to a point, and closed 

 by fusing the end. The flask is filled with distilled water, 

 and the whole apparatus sterilised. When the apparatus is 

 to be employed, the tube Asp is connected with an aspirator ; 

 for instance, a bottle filled with water and provided with an 

 outlet cock ; the cap H is taken off, and the air then passes, 

 bubble by bubble, through the opening o, through the water g, 

 and out through the cotton-wool plugs of the tube Asp. Since 

 all the germs of the air are not retained by the water when 

 the air-bubbles ascend through the latter, the cotton-wool 

 plug w is intended to catch those which get past the water. 

 When the experiment is finished, the cap H is replaced over 

 the tube R. By blowing through Asp, the liquid is made to 

 ascend in R, in order that 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 transferred, drop by drop, into a large 

 number of flasks containing sterilised broth. 



The main point here 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 several samples 

 of the water may be diluted to different degrees, 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 

 shower of rain purifies the air from bacteria to a marked 



