BACTERIA IN THE AIR. 



629 



Fio. 191. 



filters, c and b. The cap being removed and the aspirator attached, 

 the air is drawn through the water, by which suspended germs are 

 arrested ; or if not they are caught by the inner cotton plug b. The 

 sealed point of the tube B is now broken off, and the contents of the 

 flask equally divided in thirty to forty tubes containing bouillon, 

 which are placed in the incubating oven. 

 Twenty-five cubic centimetres of bouillon 

 are also introduced into the flask, and the 

 cotton plug b is pushed into it so that any 

 bacteria arrested by it may develop. If 

 one-fourth or one-fifth of the bouillon tubes 

 show a development of bacteria it is in- 

 ferred that each culture originated from 

 a single germ, and the number present in 

 the amount of air drawn through the flask 

 is estimated from the number of tubes in 

 which development occurs. 



The method adopted by Straus and Wiirtz is more convenient and 

 more reliable in its results. This consists in passing the air by means 

 of an aspirator through liquefied nutrient gelatin or agar. The ap- 

 paratus shown in Fig. 192 is used for this purpose. Two cotton 

 plugs are placed in the tube B, to which the aspirator is attached, 

 and after the determined quantity of air has been passed through the 

 liquefied medium the inner plug is pushed down with a sterilized 

 platinum needle so as to wash out in the culture 

 medium any germs arrested by it. Finally the 

 gelatin or agar is solidified upon the walls of 

 the tube A by rotating it upon a block of ice or 

 under a stream of cold water. It is now put 

 aside for the development of colonies, which are 

 counted to determine the number of germs pre- 

 sent in the quantity of air passed through the 

 liquefied culture medium. The main difficulty 

 with this apparatus is found in the fact that the 

 nutrient gelatin foams when air is bubbled 

 through it ; for this reason an agar medium is 

 to be preferred. In using this it will be neces- 

 sary to place the liquefied agar in a bath main- 

 tained at 40 C. Foaming of the gelatin is pre- 

 vented by adding a drop of olive oil before ster- 

 ilization in the steam sterilizer. But this inter- 

 feres with the transparency of the medium. 

 In the earlier experiments upon atmospheric organisms Pasteur 

 used a filter of asbestos, which was subsequently washed out in a 



FlO. 192. 



