478 BACTERIOLOGICAL ANALYSES 



VII. The Nitric Organisms. 



3. Take ten tubes of Winogradsky's solution No II, 

 number them consecutively from i to 10 and inoculate 

 with quantities of soil water similar to those enum- 

 erated in section VI step 2. Label and incubate at 

 30 C. 



4. Examine after twenty-four and forty-eight hours' 

 incubation. From those tubes that show signs of 

 growth make subcultivations in fresh tubes of the 

 same medium and incubate at 30 C. 



5. Make further subcultivations from such of those 

 tubes as show growth, and again incubate. 



6. If growth occurs in these subcultures, make 

 surface smears on plates of Winogradsky's silicate 

 jelly (vide page 198). 



7. Pick off such colonies as make their appearance 

 and subcultivate in each of these two media. 



TESTING FILTERS. 



Porcelain filter candles are examined with reference 

 to their power of holding back all the micro-organisms 

 suspended in the fluids which are filtered through 

 them, and permitting only the passage of germ-free 

 filtrates. In order to determine the freedom of the 

 filter from flaws and cracks which would permit the 

 passage of bacteria no matter how perfect the general 

 structure of the candle might be, the candle must first 

 be attached by means of a long piece of pressure tubing, 

 to a powerful pump, such as a foot bicycle pump, fitted 

 with a manometer. The candle is then immersed in 

 a jar of water and held completely submerged whilst 

 the internal pressure is gradually raised to two atmos- 

 pheres by the action of the pump. Any crack or flaw 

 will at once become obvious by reason of the stream 

 of air bubbles issuing from it. 



The examination for permeability is conducted as 

 follows : 



