On determining the Number of Micro-organisms in Air. 463 



to be aspirated, though it is of no consequence for the air of buildings 

 where the aspiration of only one-half, or at most 1 litre of air is 

 necessary, and occupies less than two minutes. The rate of aspiration 

 we have employed with our own method has been the same as with 

 Hesse tubes, viz., 1 litre in three minutes. It is not at all unlikely, 

 however, that a more rapid rate might be adopted without affecting 

 the accuracy of the results. 



Addendum. Received April 22, 1888. 



The following experiments were made for the purpose of testing 

 whether any micro-organisms pass into the exit tube or become 

 attached to the under side of the cork. 



A. As regards the Passage of Organisms into the Exit Tube. 



In these experiments, the flask was fitted up and charged with 

 jelly in the ordinary manner, except that a little jelly was also placed 

 in the bend of the exit tube. The whole was then sterilised as usual, 

 and, during the subsequent cooling, the flask was so manipulated that 

 a coating of jelly was formed over the inside walls of the exit tube, 

 keeping clear, however, of the cotton-wool plugs. Half a litre of air 

 was then drawn through each flask at the rate of 1 litre in three 

 minutes. The samples were collected in a room in which a slight 

 dust had been raised by the shaking of a door-mat. After the lapse of 

 eight days, the number of colonies counted in each flask was as 

 follows. In no case were any colonies found in the exit tube. 





Per \ litre of air. 







In flask. 



In exit 

 tube. 





Experiment I . . 



About 300 







Collected just after raising 









of dust. 



Experiment II . . 



About 200 







Collected after a few mi- 







miles' interval. 



Experiment III . . 



About 250 







Collected after a few mi- 







nutes' inteiwal. 



Experiment IT . . 



About 180 







Collected after a further in- 







terval of a few minutes. 



B. As regards the Attachment of Organisms to the Under Side of the 



' Cork. 



The flasks were charged and sterilised in the ordinary way, but 

 during cooling, after sterilisation, the flask was so manipulated as to 



