188 ATLAS OP BACTERIOLOGY. 



water. When air is to be examined a definite volume 

 is sucked through a tube of sterilized sand, the latter 

 carried into sterilized water, and plates are then 

 formed. 



(e) Agar plate cultures are made in the same way. 

 The agar should not be poured into the dishes when 

 too cool, because otherwise it coagulates at once into 

 an irregular surface ; if used when too warm, the in- 

 oculated bacteria will die 1 . In recent times it has 

 been recommended that in making agar plates the 

 nutrient medium should first be allowed to become 

 rigid in the dish, and then the mass to be examined 

 is smeared superficially upon it with a sterilized 

 platinum loop, a strip of filtering paper or a platinum 

 brush. In this way we obtain only characteristic 

 superficial colonies. 



(/) Sugar-agar-agitation cultures : The contents of 

 the tube are melted in the water-bath, then cooled to 

 about 40 ; a loopful of pure culture is then intro- 

 duced, the tube well shaken, and when it becomes 

 rigid the culture is placed in the incubating chamber. 



4. ANAEROBIC CULTURES. 



We have employed almost exclusively Buchner's 

 method, i.e., the absorption of oxygen by pyrogallic 

 acid and potash lye.* 



(a) For stick cultures : Upon the bottom of a glass 

 cylinder, which must be somewhat longer and wider 

 than a test tube, is placed a heaping teaspoonful of 

 pyrogallic acid and 20 c.c. of a three-per-cent potash 



* Sensitive varieties are said to thrive still better in a hydrogen 

 atmosphere. 



