MATERIALS. 
317 
the temperature rises above 80 the gelatin is likely to melt and spoil the 
plate. It takes about two days for the colonies to appear. After two or 
three days carefully study the plate, noting the liquefying colonies and the 
non-liquefying colonies. Note also other differences. Isolate and inocu- 
late upon agar slants several of the different colonies, including both 
liquefiers and non-liquefiers. 
Litmus gelatin or litmus agar plates should be made in the same way, and 
the appearance of a red color around some of the colonies will make it pos- 
sible to detect the acid-forming bacteria. 
No. 15. Potato Tubes. Select a large fair potato, care- .- 
fully wash and peal. With a special cutter or with a broken k^ twr 
test-tube with sharp edges, bore out some cylindrical plugs 
of potato. Cut them obliquely so as to make two wedge- 
shaped pieces of each plug, and soak in running water 
overnight. In the bottom of some large test-tubes place 
a little cotton and enough water to cover it (Fig. 63). Place 
a single potato slant in each tube and sterilize by one-half 
hour steaming upon three successive days. 
No. 1 6. Milk Tubes. Place about 10 c.c. of skim milk in 
test-tubes and sterilize for one-half hour on three successive 
days. The milk should be first tested with litmus-paper, 
and if acid, should be made neutral by adding NaOH. 
Litmus Milk. This is made as above, except that enough 
litmus solution is added before sterilizing to give a moder- 
ately blue color. 
No. 17. Fermentation Tubes. Prepare 200 c.c. of 
bouillon as described in No. 2, adding to it 2 grams lactose 
and adjust the reaction to the neutral point. To a second 
lot of bouillon add the same amount of dextrose and to a 
third lot the same amount of saccharose. After dissolving 
and filtering pour into fermentation tubes, enough to fill the 
closed arm and half the bulb (Fig. 64). A dozen or more 
tubes of each of these bouillons should be prepared and 
labeled. Sterilize by steaming on three days. If gas col- 
lects in the closed arm remove by tilting the tube. 
No. 1 8. Testing Characters of Bacteria. Several isolated and purified 
cultures of bacteria have been prepared in No. 7. After having prepared 
the several culture media above described, use'them as follows: Make a 
fresh agar slant from each purified bacteria culture and allow to grow 
about 24 hours. If possible use a culture of B. coli for one of the series of 
tests. Then inoculate with a small quantity of the growth the following. 
a. Two agar slants, b. One gelatin stab. This is made by dipping a 
straight platinum needle into the bacteria and then thrusting it straight 
into the gelatin of a gelatin tube, in the middle of the tube, and forcing the 
needle to the bottom, carefully withdrawing without disturbing the gelatin. 
c. A fermentation tube of each of the three sugars, d. Two milk tubes. 
/. Two litmus milk tubes, g. Two potato tubes, inoculating the potato]on 
FIG. 63. 
Potato tube. 
