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- 

 croorganisms 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 in diameter. 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 2 , are used, the lower 

 one of which serves as a control to prove that 

 all microorganisms present in the air have been 

 arrested by the upper one. The upper filter is 

 protected, until the aspirator attached to the 

 tube h 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 

 its physical characters. Loaf sugar is pulver- 

 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 out and hermetically sealed. Two 

 cotton plugs are placed at the lower portion of the tube, at a and at b. 



FIG. 191. 



