464 On determining the Number of Micro-organisms in Air. 



allow the jelly to form a thin coating over the under side of the cork. 

 Half a litre of air was then drawn through each flask at the rate of 

 1 litre in three minutes. The samples were collected as before, 

 except that the dust raised was not nearly so great. After nine days, 

 the following number of colonies had developed on the jelly in the 

 flasks, but not a single one was observed on the under side of the 

 cork : — 





Per | litre of air. 





In flask. 



On cork. 



Experiment I 

 Experiment II . . 



57 

 23 







.0 ; 



Collected just after raising of 

 dust. 



Collected after an interval of a 

 few minutes. 



The above results show, therefore, that, with an aspiration of 

 1 litre of air in three minutes, all the organisms are deposited on the 

 jelly at the bottom of the flask, and that none reach the cork or exit 

 tube. This result is probably due not only to the action of gravity, 

 but also to the initial velocity, with which the organisms leave the 

 mouth of the entrance tube and enter the flask, being such as to 

 project them on to the surface of the jelly at the bottom of the flask, 

 where they stick and have not the chance of rising again. 



