142 BACTERIA 



the culture. An old-fashioned one was to plate out the 

 culture and protect it from the air by covering it with a 

 plate of mica. A more serviceable mode is to inoculate, 

 say, a tube of agar with the anaerobic organism, and then 

 pour over the culture a small quantity of melted agar, which 

 will readily set, and so protect the culture itself from the 

 air. Oil may be used instead of melted agar. Another 

 mechanical method is to make a deep inoculation and then 

 melt the top of the medium over a bunsen burner, and thus 

 close the entrance puncture and seal it from the air. 



5. Absorption of Oxygen by an Aerobic Culture. This 

 method takes advantage of the power of absorption of cer- 

 tain aerobic bacteria, which are planted over the culture of 

 the anaerobic species. It is not practically satisfactory, 

 though occasionally good results have been obtained. 



6. Lastly^ there is the Air-pump Method. By this means 

 it is obviously intended to extract air from the culture and 

 seal of it in vacuo. The culture tubes are connected with 

 the air-pump, and exhausted as much as possible. 



Of these various methods it is on the whole best to choose 

 either the hydrogen method, the vacuum, or the plan of 

 absorption by grape-sugar or pyrogallic. In anaerobic plate 

 cultures grape-sugar agar plus 0.5 per cent, of formate of 

 soda may be used. The poured inoculated plate should be 

 placed over pyrogallic solution under a sealed bell-glass and 

 incubated at 37° C. Pasteur, Roux, Joubert, Chamberland, 

 Esmarch, Kitasato, and others have introduced special ap- 

 paratus to facilitate anaerobic cultivation, but the principles 

 adopted are those which have been mentioned. 



THE QUALITATIVE ESTIMATION OF BACTERIA IN THE SOIL 



We may now turn to consider the species of bacteria 

 found in the interstices of soil. They may be classified in 

 five main groups. The division is somewhat artificial, but 

 convenient : 



