316 LABORATORY WORK. 
see if the bacteria are still stained blue or have been decolorized. Differ- 
ent species differ in this respect. If they are still stained they are Gram- 
positive; if decolorized, they are Gram-negative. 
No. ii. Microscopic Study of Yeast. Rub up a bit of an ordinary yeast 
cake in a little water. Place a dilute drop on a slide and proceed to stain 
exactly as above described for bacteria. Study with 1/12 immersion lens 
and compare with yeast as to size and shape. Look over the specimen and 
find some cells that show buds. 
No. 12. Gelatin Culture Medium. a. Weigh out the same ingredients as 
directed in No. 2, omitting the agar. After the mixture has dissolved add 12 
per cent, of first-grade gelatin. Allow to soak till soft and almost melted. 
Weigh the dish with its contents. Bring to a boil slowly and boil for five 
minutes and add water to restore original weight. 
b. Determine the acidity and bring the reaction to 1.5 as described in 
No. 2. 
c. Boil 15 minutes. Cool and add the white of an egg dissolved in a 
little water. Heat slowly to boiling and allow to boil gently till the egg 
is coagulated and the liquid clear ; usually about 1 5 minutes. Replace the 
evaporated water. 
Heat once more to boiling and filter through absorbent cotton. Place 
10 c.c. in each of about fifty tubes and the rest in a flask, plugging both 
with cotton. Sterilize in the steam sterilizer for 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 steam 
again. Gelatin cannot be sterilized in the autoclave satisfactorily, since 
too high a heat will prevent its subsequently hardening when cooled. 
Too long boiling will in the same way ruin the gelatin. 
No. 13. Litmus Gelatin and Litmus Agar. These are used chiefly to 
detect acid-producing bacteria. Make agar or gelatin in the manner al- 
ready described, except that i per cent, lactose is added, and 1.5 per cent, 
agar instead of 1.2 per cent., or 15 per cent, gelatin instead of 12 per cent. 
These are to be filtered in the usual way, and are known as lactose agar 
and lactose gelatin. 
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 it a blue color. Tube 
the medium as usual and sterilize. 
Instead of using -the litmus solution azolitmin may be used. This 
is a powder that may be dissolved in a little alcohol and sufficient added to 
the lactose agar or lactose gelatin to give a blue color. Its use is simpler 
and in some respects better than litmus solution. 
No. 14. Gelatin Plates from Milk. Procure some milk that is not more 
than six hours old. Dilute 1000 times, as directed in No. 4. Place i /2 c.c. 
of the dilution in two Petri dishes and i 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 
