ORIGINAL METHODS 



863 



and the flask put on one side : the same process is repeated with a number of flasks. 

 In a short time the medium in some of the flasks will become cloudy, and from the 

 number which show this turbidity the number of organisms contained in the air 

 can be roughly deduced. 



Thus, for example, suppose that 50 flasks each containing approximately 500 c.c. of 

 air were used and that 20 of them became cloudy : the calculation would be as follows : 

 25 litres of air have given 20 organisms ; 1 cubic metre therefore contains very approxi- 

 mately f x 1000 that is 800 organisms. 



The method necessitates the use of a great deal of material and is cumbersome 

 and from the practical point of view, impossible. 



II. Konh's method. Koch's method consists in exposing to the air for different 

 periods of time a number of gelatin plates and studying the colonies which subse- 

 quently develop on them. This method does not allow of quantitative estimations. 



FIG. 409. 



FIG. 410. 



FIGS. 409, 410. An agar plate and a gelatin plate exposed to the air side by 

 side and then incubated at 37 C. and 20 C. respectively. Note the greater 

 number of bacteria on the former and of moulds on the latter. 



III. Hesse's method. Hesse's method which has the advantage of being simple 

 is based upon the principle of the aeroscope. Unfortunately the results obtained 

 are only approximate. 



Take a piece of glass tubing 4-5 cm. in diameter and 50-70 cm. long (fig. 411). 

 Plug one end of the tube with an india-rubber plug through which a piece of glass 

 tubing plugged with wool at the outer end is passed. 



FIG. 411. Hesse's tube. 



Cover the other end with two pieces of india-rubber one over the other the inner 

 being perforated with a hole about 1 cm. in diameter. Sterilize the apparatus and 

 then pour about 50 c.c. of liquefied sterile gelatin into the tube through the perforated 

 piece of india-rubber. Replace the second sheet of india-rubber at once and keep 

 the tube in an horizontal position until the gelatin has set. The gelatin should form 



