348 AUGUSTO BONAZZI 



found in the form of volatile acids, failed to reveal their presence 

 when the cultures were distilled in presence of tartaric acid. 

 Nevertheless, whatever these compounds are they do not seem 

 capable of stopping the development of Azotobacter as is shown 

 in the following experiment. 



Experiment 71. A series of flasks containing each 25 cc. of a 

 glucose solution, Ca(N03)2 and CaCOs was inoculated with a 

 pure culture of Azotobacter chroococcum and incubated at 30°C. 

 One flask was left uninoculated as a control. After the neces- 

 sary period of incubation had elapsed the solutions were acidified 

 until complete solution of the carbonate, allowed to stand in 

 this acidified condition for a short time and then filtered with 

 suction, through a crucible filter prepared according to the 

 accompanying illustration (fig. 5). 



Fig. 5. a, perforated glazed crucible; b, glass wool; c, washed, digested, ignited 

 quartz sand; d, asbestos; e, packed and burnished platinum sponge. 



The solutions passing through the filter were perfectly clear 

 showing that the cells were completely retained by the filter. 

 Careful washing with water, in small quantities at a time, insured 

 complete removal of the retained sugar. By this procedure it 

 was possible to separate the cells from the surrounding medium 

 so as to form a conception of the ratio S:c where S is the sugar 

 consumed and c the weight of the cells produced, reckoned in 

 terms of cellular substance dried at 110°C. in vacuum, over 

 P2O5. 



One of the above mentioned cultures, no. 5, was not filtered 

 but received instead the addition of 2 cc. of a 12.5 per cent 

 glucose solution under aseptic conditions; allowed to incubate 

 for a longer period of time it was then subjected to the same 



