LABORATORY WORK 303 



each, using a loopful of the milk in a water blank and inoculating; 

 gelatin tubes with a small loopful of the water blank dilution. 

 After proper growth examine the colonies in the different plates 

 and compare with each other. What does the experiment teach? 

 If time permits, the colonies may be isolated, purified and further 

 studied. 



17. Dust Plates. — Pour 12 tubes of melted gelatin into petri 

 dishes ; cover and allow to harden. 



a. During the milking of a cow hold one of the above gelatin 

 plates under the udder of the cow, close to the milk pail; remove 

 the cover so as to allow any dust particles to fall on the gelatin, for 

 half a minute. Replace the cover and carry to the laboratory. 



b. Place 3 of the flasks at different places in a cow barn 

 several hours after feeding. Remove the covers for 3 minutes. 

 Replace covers and carry to laboratory. 



c. Repeat b ten minutes after feeding the cows with hay. 



d. Repeat c after feeding with other foods. 



e. Expose one plate the same length of time out of doors. 

 After the above plates have developed count the colonies on 



all the plates and compare. 



18. Bacteria in Hay. — Soak a little hay in warm (not 

 hot) water for an hour. Shake well and inoculate three litmus 

 gelatin tubes, the first with one, the second with two and the third 

 with three loops full of the hay infusion. Pour into petri dishes, 

 incubate and study as usual. Compare the numbers of liquefiers 

 and non-liquefiers. Are there any acid-forming colonies ? 



19. Bacteria in Grain. — Repeat the above experiment, using 

 some grain feed instead of hay. 



20. Value of Cleaning Milk Bottles. — Procure 4 glass bottles 

 in which milk has been standing for a day; ordinary unwashed 

 milk bottles are best. 



a. Wash one with cold water. 



b. Wash one with hot water and soap. 



