On determining the Number of Micro-organisms in Air. 459 



adhered to the sides of the entrance tube, even when the latter was 

 coated with jelly, so that under ordinary conditions the number so 

 adhering would probably be very much less. This apparent source of 

 error, therefore, may be entirely neglected when the width of the 

 entrance tube is not less than that prescribed. 



In order to test the quantitative accuracy of the method, a number 

 of comparative experiments were made by collecting samples of air 

 simultaneously in the flasks and in Hesse tubes, placed side by side. 

 On p. 458 is a table of the results obtained in this way. In comparing 

 these results it must not be forgotten that, even when two Hesse 

 tubes are compared the one against the other, it is only occasionally 

 that identical numbers are obtained in each tube. Thus one may get 

 six in one tube and eight in the other, or twenty in one tube and 

 twenty-three in the other, and so on, the difference varying according 

 to the total number of micro-organisms present. 



From the above table it will be seen that in nearly all cases the 

 number of micro-organisms (both bacteria and moulds) in the tube 

 and in the flask correspond almost exactly. In Nos. 6, 7, 9, 15, 16, 

 and 19, however, this is very far from being the case, for in each of 

 these the flask method gave very much lower results than the Hesse 

 tube. Of these six non-concordant experiments, four were made in 

 outside air, and the other two in schoolrooms in which there was a 

 considerable draught, for the day being warm, the windows and doors 

 were all open. 



Now Dr. Percy Frankland (luc. cit.) has conclusively proved that 

 Hesse's method does not give reliable results for outside air, except 

 on calm days. He made a number of experiments in which a control 

 tube was used side by side with the aspirated tube, and in this way 

 he was able to obtain a rough idea of the number of micro-organisms 

 which gain access to a Hesse tube, irrespective of aspiration. In 

 illustration of this we may quote a few of his results : — 



Table III. 



No. 



State of wind 



Vol. of air 

 taken. 



Mic ro - organ i sm s 

 in aspirated tube. 



Micro-organisms in 

 non -aspirated tube. 



1 





12 litres. 



158 



54 



2 



Slight 



12 „ 



12 



3 



3 



Moderately strong . . . 



12 „ 



53 



11 



4 



Moderately strong . . . 



12 „ 



114 



34 



5 



Moderate, but variable 



12 „ 



49 



29 



6 





11 » 



52 



15 



7 





10 „ 



75 



15 



8 





12 „ 



78 



48 



9 



Slight 



12 „ 



72 



27 



2 m 2 



