LABORATORY WORK. 
its surface only. Place one of the agar tubes, one of the milk tubes, litmus 
milk tubes, and potato tubes in the incubating oven at 98. Place all others 
at room temperature. Allow to grow several days, examining each day, 
For each species of bacterium note the following points: 
Agar Slants; Color. Is growth thick or thin, moist or dry; does it 
spread ? 
Morphology. With the microscope study stained specimens; note 
shape formation of chains spores. Determine motility. 
Gelatin Stab. Note liquefaction, needle growth, surface growth. 
Compare growth with Fig. 3 5. 
Bouillon. Note turbidity, scum, sedi- 
ment. 
Milk Tubes. Note at 98 and 70 the 
development of acid as shown by its action 
on litmus, curdling, separation of whey, 
appearance of gas bubbles, subsequent 
softening and solution of the curd, called 
digestion. 
Potatoes. At 98 and 70, note color, 
abundance of growth, texture of growth, 
discoloration of potato. 
Fermentation Tubes. After several 
days note whether gas has collected in the 
closed arm. If not, record the bacterium 
as forming no gas. Test the' liquid in the 
bulb with litmus-paper to see if acid. If 
gas is formed in any tube place a mark 
on the tube at the top of the liquid to 
mark the amount of gas. Then fill the 
bulb completely with a 2 per cent. NaOH 
solution. Place the thumb over the open- 
ing of the bulb so that there will be no gas 
bubble between the thumb and the surface of the liquid. Invert the 
tube, allowing the gas to flow into the bulb, and by turning back and 
forth mix the gas with the NaOH solution, keeping the thumb in position 
all the time. After mixing turn the tube once more so that all the gas 
will be in the closed arm. Remove the thumb and it will usually be found 
that the level of the liquid in the closed arm rises, because some of the gas 
has been dissolved in the NaOH. This dissolved gas is CO 2 . By deter- 
mining the level of the gas before and after the treatment the proportion 
of CO 2 to the undissolved gas may be determined. This is called the gas 
ratio. 
Tabulate the characters of the different species of bacteria tested, 
determining whether any two of them are alike in all respects. It is 
by such characters that bacteria are described. To determine species is 
too difficult for a beginner. 
No. 20. Test for Indol. Make the following: 
FIG. 64. Fermentation tube with 
closed arm containing gas. 
