3 02 CLEAN MILK 



a clean brush and then with a damp cloth. Wash the udder and 

 especially the teats with a 10% borax solution. After drawing about 

 a dozen jets on to the ground, remove the plug from the flask and, 

 placing its mouth very close to the teat, milk it about half full, and 

 replace the cotton plug. Carry at once to the laboratory and make 

 a litmus gelatin plate immediately, diluting 10 times. After 2 or 

 3 days' growth, count the colonies. Are there any acid colonies 

 of the Bact. lactis acidi present? If time permits, isolate the col- 

 onies ; purify and determine the general class of bacteria. 



Allow the rest of the milk to stand at about jo° to see if it 

 will sour normally. 



14. Effect of Temperature on Species of Bacteria in Milk. — 

 Procure some fresh milk and make several litmus gelatin plates, 

 diluting 100 times. Divide the milk in three lots, placing one at 

 about 50 , one at 70 , and one at 98 . At the end of 24 hours 

 make plates from each, diluting that kept at 50 2,000 times, that 

 at yo° 10,000 and 100,000 times, and that kept at 98 500,000 

 times. After another 24 hours, make some plates from the sample 

 at 70 , diluted 10,000 times. Allow all plates to grow at room tem- 

 perature until the colonies are well developed. Determine the total 

 number of bacteria in each sample, and the number and percentage 

 of each kind of bacteria that can be clearly distinguished by its 

 colony. Compare the numbers and percentages of the different 

 species at the different temperatures. 



15. Bacteria of the Air. — Place one dozen tubes of steril- 

 ized milk — with cotton plugs removed — in various places around 

 the barn, dairy, house and laboratory. Leave undisturbed for 6 

 hours. Then replace the plugs, carry all to laboratory, and place at 

 about 70 . AVatch for several days and notice whether all appear 

 to undergo the same kind of fermentation. Do any of them sour 

 normally ? Do any remain unchanged ? 



16. Types of Bacteria from the Air. — After the tubes in 

 No. 15 show signs of fermentation make a gelatin plate from 



