322 LABORATORY WORK. 



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. Put a 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 appear- 

 ance. 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. Remove a bit of the mycelium, 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 com- 

 pare in size with bacteria and yeasts? 



No. 35. Germination of Mold Spojres. Melt two or three agar tubes and 

 two gelatin tubes and add a few drops of HC1, 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 micro- 

 scope and note that spores have been planted on the surface. Set in a warm 

 place and examine in twe'nty-four hours to see if the spores are germi- 

 nating. 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 steril- 

 izing inoculate, two with a little yeast (from a yeast cake) and set in a 

 warm place. Examine in eight to twelve hours. Note the gas bubbles 

 rising in the closed arm. Remove a little of the sediment and examine 

 under the microscope (both stained and unstained). Note the clusters of 

 budding yeast cells. How do they compare with the cells in the yeast 

 cake? After the arm is about half-full of gas, test with NaOH as described 

 in No. 1 8. By the amount of gas dissolved by the NaOH determine how 

 much of the gas is CO 2 Does any fermentation occur in the sterilized 

 tubes? In the original unsterilized and uninoculated tubes? If fermenta- 

 tion occurs in the latter examine with a microscope to see if yeast is present. 



