BACTERIA IN THE AIR. : 619 
filters, cand 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 0 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 Fie. 189, 
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. 190 is used for this purpose. Two cotton 
plugs are placed in the tube B, to which the aspirator is attached, 
and afterthe 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 
Fie. 190. 
