On determining the Number of Micro-organisms in Air. 461 



organisms gained entrance to the flasks irrespective of aspiration, 

 corresponding experiments being made simultaneously with Hesse 

 tubes. For this purpose a pair of flasks and a pair of Hesse tubes 

 were simultaneously exposed to the outside air for the same length of 

 time, but without aspirating air through any of them. The exit tube 

 (which in an ordinary experiment is connected with the aspirator) of 

 one of each pair of flasks and tubes was stoppered, and the exit tube 

 of the other flask and tube left unstoppered. The entrance to each 

 flask and tube was of course left open. The total number of colonies 

 obtained in each case were as follows, the numbers in brackets being 

 the number of moulds : — 



Table V. 



No. 



State of wind. 



Time 

 of ex- 

 posure. 



Hesse 



tubes. 



Flasks. 



Stoppered. 



Un- 

 stoppered. 



Stoppered. 



Un- 

 stoppered. 



1 





| hour 



2 [2] 



23 ri] 





2[iJ 



2 





1 a 





"o 







3 



Gentle 



1 » 



6 [5] 













4 



Moderately strong 















and variable. . . . 



^ jj 



8 [6] 



12 [§] 





o 



5 



Rather strong and 

















i 



2 » 



sro] 



12 [0] 







1[0] 



6 



Rather strong and 

















i 



2 » 



45 [2J 



33 [1] 







1[0] 



Thus out of ten flasks exposed to the air for half to one hour, 

 only three were contaminated, and these only very slightly, and on 

 very windy days, whereas the Hesse tubes were considerably contami- 

 nated. It is thus seen that the flask method, unlike the Hesse tube 

 method, is practically free from vitiation by aerial disturbance. 



We can fully confirm Dr. P. Frankland's statement that Hesse's 

 method gives good results in cases where the air is still and free from 

 draughts, as in most inside buildings and outside on calm still days, 

 for under these conditions Hesse's method agrees remarkably well 

 both with Frankland's process and with our own ; whereas in a dis- 

 turbed atmosphere, as in outside air on windy days, or in buildings 

 where a strong draught prevails, Hesse's method gives results which 

 are considerably in excess of those obtained either by Frankland's 

 method or by our own. 



The following are the chief advantages of the new method : — 



(1.) It possesses, in common with Hesse's and Frankland's processes, 

 the advantages of a solid nutrient medium. 



