346 LABORATORY WORK 
curdled. Dilute it 100,000 times (two 99 c.c. and one 9 c.c. water blanks) and make 
plates in litmus gelatin or litmus agar. Incubate at 70° for two or three days and 
count the number of bacteria. Count the number of acid colonies. 
No. 31. Bacteria in Cream.—Make plates as in No. 30 from fresh cream, dilut- 
ing 1,000 times and using litmus gelatin and litmus agar. Make other plates from 
some ripened cream in gelatin and in agar and diluting 100,000 times. Incubate at 
7o° for three days and compare the plates from the two lots of cream as to number of 
bacteria and relative proportion of acid bacteria. 
No. 32. Efficiency of Pasteurization.—Obtain some milk ten to fifteen hours old. 
Make three plates as described in No. 30. Place the milk in a jar, and heat in 
water to 140°, keeping it at that temperature for one-half hour. Make from it a 
second series of plates. Incubate plates at 98° for twenty-four hours. Count the 
number of colonies in the two sets of plates. 
No. 33. Bacteria on Hay.—Place a handful of hay in water and warm slightly 
for ten minutes. The water should be hardly more than luke warm. This is called 
a hay infusion. Puta platinum loop of the infusion into a water blank and inoculate 
a loopful of this dilution into an agar plate. Incubate at 98° for one day and count 
the bacteria. 
No. 34. Study of Common Molds.—Place under a bell glass, or in some other 
closed box which will prevent evaporation, some pieces of bread, slightly moistened, 
two or three pieces of cheese, and some slices of a lemon. Keep these in a warm 
place for a few days until molds make their appearance. Examine day by day until 
they become covered with spore masses. Note the mycelium, its fineness of texture 
and the color of the spore masses. Determine, if possible, whether the mold masses 
on the different objects are the same or different species. Removea bit of the myce- 
lium, place in a drop of water on a slide under a cover-glass and examine under the 
microscope. Sketch the threads and their method of branching. Place some of the 
spores under the microscope. Sketch. How do they compare in size with bacteria 
and yeasts? 
No. 35. Germination of Mold Spores.—Melt two or three agar tubes and two 
gelatin tubes and add a few drops of HCl, just sufficient to make the medium slightly 
acid. Pour out in Petri dishes and allow to harden. With a platinum needle 
transfer the smallest possible quantity of spores from one of the molds and touch 
the surface of the agar and gelatin in several places with the tip of the needle. 
Examine with low-power microscope and note that spores have been planted on the 
surface. Set in a warm place and examine in twenty-four hours to see if the spores 
are germinating. Sketch a germinating spore. Allow to grow till spores are 
formed, studying daily with microscope. 
No. 36. Yeast and Fermentation.—Grind up a few apples in a meat-cutter 
squeeze the juice through cheese-cloth and then filter through filter-paper. Fill 
six fermentation tubes with the juice, filling the closed arm full and the bulb half- 
full; plug with cotton. Set two of them in a warm place. Sterilize the other four 
by steaming for half an hour. After sterilizing inoculate, two with a little yeast 
(from a yeast cake) and set in a warm place. Examine in eight to twelve hours. 
