86 BACTERIA IN THE AIR 



wool. The gelatine having set, the flask was incubated at 20° C. till no 

 further colonies of bacteria or moulds developed. Some of the chief 

 results were as follows : — 



* The ventilation of this large room was considerable, but having the effect of keeping dust in 

 suspension rather than expelling it from the room. Three tests made here were all in the same work- 

 place, differing only in degree of dust present. 



(ii.) In 1902 the writer made some observations in Finsbury on the 

 number of bacteria to be found in the air of underground bakehouses. 

 Four were selected, and the degree of carbonic acid gas was estimated 

 by Pettenkofer's method, and examinations were made as follow of 

 the bacteria pollution. In each of these bakehouses, whilst work 

 was going on, three agar-plates (of 9 '6 inches area each) were exposed 

 for thirty minutes. One plate was placed on the floor, one on the 

 table or trough where the bread was being made, and one on a shelf 

 near the ceiling.- After exposure for thirty minutes the plates were 

 re-covered and incubated at blood-heat (37° C), for exactly twenty- 

 two hours. All the plates then showed abundant growth. Doubtless 

 if the plates had been incubated for forty-eight hours, or three or 

 four days, there would have been a greater growth of colonies, and 

 it is probable also that if some of the plates had been placed at room 

 temperature certain bacteria would have grown which did not appear 

 at blood-heat in twenty -two hours. It is not suggested that these 

 plates provide an adequate record of the bacteria present in the air 

 of these bakehouses. The object was merely to obtain a comparative 



