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 the 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 : 
(3) Growth in hydrogen. 
(4) Growth in carbon dioxide. 
(5) Growth in vacuo, various degrees of ex- 
haustion. 
(6) Growth in vacuo, 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 
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 
*F 1c. 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. 
