LABORATORY WORK 341 
twenty minutes. Set aside for twenty-four hours and steam a second time; this 
time allow half an hour steaming. Set aside for another twenty-four hours and 
steamagain. Gelatin can also be sterilized in the autoclave, but too high a heat will 
prevent its subsequently hardening when cooled; hence only a 15-minute steriliza- 
tion at 15 pounds pressure may be used. Too long boiling will in the same way 
ruin the gelatin. 
No. 14. Litmus Gelatin and Litmus Agar.—These are used chiefly to detect 
acid-producing bacteria. Make agar or gelatin in the manner already described, 
except that 1 per cent. lactose is added, and 1.5 per cent. agar 
instead of 1.2 per cent., or 15 percent. gelatin instead of 12 per Ae 
cent. These are to be filtered in the usual way, and are known as NN \' ae 
lactose agar and lactose gelatin. Yaee 
Prepare a litmus solution by soaking 50 grams of dry litmus . 
cubes with 250 c.c. water. Soak for twenty-four hours and filter 
through filter-paper. Add enough of this to the agar or the gelatin 
to give ita blue color. Tube the medium as usual and sterilize. 
No. 15. Gelatin Plates from Milk.—Procure some milk that is 
not morethan six hours old. Dilute roco times, as directed in 
No. 4. Place 14 c.c. of the dilution in two Petri dishes and 1x 
c.c. in two others. Pour into each the melted gelatin from 
one of the gelatin tubes prepared in No. 12. Thoroughly mix 
by gentle agitation and place in a cool place to harden. Set 
aside at a temperature of about 70° for the bacteria to grow. If 
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 inoculate 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 possible to detect the acid-forming bacteria. Fic. 62.—Po- 
No. 16. Potato Tubes.—Select a large fair potato, carefully  tato tube. 
wash and peal. With a special cutter or with a broken 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. 62). Place a single potato slant in each tube and 
sterilize. 
No. 17. Milk Tubes.—Place about ro 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 moderately blue color. 
