2 BACTERIOLOGICAL APPARATUS 
Durham tube was more efficient but with the higher dilutions the Smith 
tube gave more consistent results. Browne and Ackman (1917) and 
Browne (1917) have stated that the following factors cause variations 
in the amounts of gas produced in lactose peptone bile and lactose 
broth: (1) Temperature; (2) Time of incubation, (8) Initial reaction 
of the culture medium; (4) Length of inverted vial, (5) Source of bile; 
(6) Absorption of formed gas. 
Graves (1917) inverts the small inner tube over a wooden rod which 
lifts the tube a distance from the bottom of the outer tube. Itis claimed 
that certain advantages obtain in this procedure. 
Fig 1—Types of Fermentation Tubes. 
A, Brown's, B,Smith’s C,Smith’s, without foot, D, Graves’ 
Modification of the Durham Fermentation Tube 
Petri Dishes. These should be about 10 cm. in diameter, thus 
having an area of about 65sq.cm. LHither glass or porous tops may be 
used. The porous tops devised by Hull prevent the spreading colonies 
which appear so often on Petri dishes in humid weather. Very often 
it 1s difficult to secure Petri dishes with flat bottoms. This is essential 
to an even distribution of bacteria over the plate and a uniform thick- 
ness of medium. 
Dilution Bottles. These may be of various types and shapes but 
should contain the amount of dilution water when about half full. This 
allows ample room for thorough shaking of the diluted sample. Bottles 
