54 



BACTERIA IN RELATION TO PLANT DISEASES. 



beef-bouillon and steamed every twenty-four hours for seven or eight days, a large 

 bubble being tilted out each time and appearing just as regularly during the next 

 steaming. Naturally, no strict anaerobe would grow in such a tube and every 

 aerobe would appear to be a facultative anaerobe. The neck of the fermentation- 

 tube should be as narrow as consistent with filling and cleaning. All wide-necked 

 tubes should be discarded. The behavior of the closed end with reference to the 



absorption of air may be tested by adding litmus- 

 water and 5 per cent grape-sugar to tlie bouillon. 

 On steaming, the litmus is reduced. If there 

 is no air in the closed end the litmus remains 

 reduced, while in the open end exposed to the air 

 it soon oxidizes back to its original color. 



Other things to be observed are : 



( j) Growth in hydrogen. 



(<f) Growth in carbon dioxide. 



(5) Growth in vacua, various degrees of ex- 

 haustion. 



(6} Growth in vacua, remnant of oxygen ab- 

 sorbed by the mixture of caustic potash and pyro- 

 gallol (same as pyrogallic acid). 



(7) Growth in nitrogen (air with the oxygen 

 absorbed, normal air-pressure). 



The hydrogen and carbon dioxide, which are 

 required in considerable quantities, may be gen- 

 erated in Kipp gas-generators. There is a choice 

 in generators. The writer has not found any kind 

 which is entirely satisfactory. The one which has 

 given the least trouble is shown in fig. 50. The 

 objection to this generator is the large volume of 

 dead acid which soon accumulates at the bottom. 

 The accumulation of dead acid is entirely obvi- 

 ated in the de Koninck generator, but the writer 

 has only recently obtained this apparatus and has 

 not yet had enough experience with it to speak 

 unqualifiedly. It furnishes a large amount of gas 

 and its generation may be stopped very quickly, 

 but the acid chamber is inconveniently bulky (10 

 liters) and in case of breakage a destructive flood 



Fig. 50.* 



would be poured out into the laboratory. To avoid this the apparatus should be 

 set into a deep enameled iron pan. The action of the apparatus depends on the fact 



*FiG. 50. Kipp gas-generator for making carbon dioxide or hydrogen. When not in use the 

 pressure of the gas forces the acid off the marble or zinc (in the middle compartment) and stops 

 its evolution. Much reduced. 



