On determining the Number of Micro-organisms in Air. 457 



an even layer of gelatine solidifies at the bottom of the flask. On 

 taking the flask out of the steriliser it is generally necessary to care- 

 fully rinse the jelly round the sides of the flask so as to take up any 

 steam which may have condensed there and which might subsequently 

 collect in drops and run down on to the colonies and inoculate the rest 

 of the jelly. 



In doing this care should be taken to avoid frothing of the jelly. 



In taking a sample of air the aspirator is attached to the exit- 

 tube 0, and the india-rubber tube and stopper B removed from the 

 end of A. A known volume of air is then drawn through the flask, 

 after which the stopper is replaced. As the air passes through the 

 flask the micro-organisms settle on the jelly, and in the course of a 

 few days develop into colonies and may be counted. If there are a 

 large number of micro-organisms present the bottom of the flask may, 

 for convenience in counting, be marked out into squares with ink. 

 The rate of aspiration we have employed is the same as in Hesse's 

 process, viz., about 1 litre in three minutes. Usually the micro- 

 organisms are deposited more or less directly under the lower end of 

 the entrance tube, while none are deposited on the sides of the flask, 

 even though the latter be coated with jelly, which would seem to 

 indicate that no micro-organisms pass over into the exit tube. 



At first sight it seemed very likely that on account of the air having 

 to pass through an entrance tube 8 inches long, a number of the 

 micro-organisms might adhere to the side of the tube and never reach 

 the jelly, so that the results obtained would be too low. In order to 

 ascertain whether this was the case or not, a number of flasks were 

 prepared in which the inside of the entrance tube was coated with a 

 thin layer of jelly. The samples of air were then taken in the usual 

 way. and after sufficient time had been allowed for the development 

 of the colonies, the number in the flask and in the entrance tube were 

 counted, with the following results : — 



Table I. 



No. 



Circumstances. 



Yol. of air 

 taken. 



No. of colonies 

 in flask. 



No. of colonies 

 in entrance tube. 



1 



Dusty air 



400 c.c. 



287 



3 



2 





500 „ 



145 



1 



3 





500 „ 



At least 300 



4 



Unfortunately we omitted to count the colonies in No. 3 for a day 

 or two, when it was found that a number of them had run together, 

 but there were at least 100, and probably many more. The above 

 results show that only about 1 per cent, of the micro-organisms 



