620 BACTERIA‘ IN THE AIR. 
culture liquid. A filter of this kind washed out in liquefied gelatin 
or nutrient agar would give more satisfactory results, as the culture 
medium could be poured upon plates or spread upon the walls of a 
test tube and the colonies counted in the usual way. Petri prefers 
to use a filter of sand, which he finds by experiment arrests the mi- 
croérganisms suspended in the atmosphere, and which is subsequently 
distributed through the culture medium. The sand used is such as 
has been passed through a wire sieve having 
openings of 0.5 millimetre indiameter. This is 
sterilized by heat, and is supported in a cylin- 
drical glass tube by small wire-net baskets. The 
complete arrangement is shown in Fig. 191. 
Two sand filters, c, and c,, are used, the lower 
one of which serves as a control to prove that 
all microérganisms present in the air have been. 
arrested by the upper one. The upper filter is 
protected, until the aspirator attached to the 
tube fh is put in operation, by a sterile cotton 
plug, not shown in the figure which represents 
the filter in use. Petri uses a hand air pump as 
an aspirator, and passes one hundred litres of 
air through the sand in from ten to twenty 
minutes. The sand from the two filters is then 
distributed in shallow glass dishes and liquefied 
gelatin is poured over it ; this is allowed to sol- 
idify and is put aside for the development of 
colonies. The principal objection to this method 
is the presence of the opaque particles of sand 
in the culture medium. This objection has been 
overcome by the use of soluble filters, a method 
first employed by Pasteur and since perfected 
by Sedgwick and by Miquel. The most useful 
material for the purpose appears to be cane 
sugar, which can be sterilized in the hot-air oven 
at 150° C. without undergoing any change in 
C its physical characters. Loaf sugar is pulver- 
Fic. 191. ized in a mortar and passed through two sieves 
in order to remove the coarser grains and the 
very fine powder, leaving for use a powder having grains of about 
one-half millimetre in diameter. This powdered sugar is placed 
in a glass tube provided with a cap having a ground joint and a cot- 
ton plug to serve as an air filter (A, Fig. 192), or in a tube such as is 
shown at B, having the end drawn outand hermetically sealed. Two 
cotton plugs are placed at the lower portion of the tube, at a and at b. 
