286 BACTERIA, YEASTS, AND MOLDS 



of agar. Into one tube of each place one cubic centimeter of the 

 diluted milk ; into a second tube of each place one half of a cubic 

 centimeter, and into a third a single drop. Mix thoroughly, pour 

 into petri dishes as usual, harden, and set aside for growth. If 

 possible, count the number of bacteria on the plates and estimate the 

 number per cubic centimeter (a single drop is about one fifteenth of 

 a cubic centimeter). The number will sometimes be too large to 

 make this possible. 



59. Effect of Temperature upon Milk. Fill six test tubes full of 

 milk. Place two of them in an ice chest, two at ordinary room tem- 

 perature, and two close to a stove or radiator where the temperature 

 is very warm. Examine at intervals of three or four hours and note 

 the time at which the tubes become sour and curdle. Determine, if 

 possible, whether there is any difference in the appearance or smell 

 of the curdled milk in the three samples. 



60. Effect of boiling Milk. Fill two test tubes one third full of 

 milk. Place one of them in water and allow the water to boil briskly 

 for five minutes. The second one is not to be boiled. At the close 

 of the boiling plug both test tubes with cotton and set side by side 

 in a warm place. Examine each day and notice the difference in 

 the changes that take place in the milk. One sample will probably 

 sour quickly ; the other will keep very much longer and will not sour, 

 even after many days, although it will spoil. Test both samples with 

 litmus paper, after they have spoiled, to see if both are acid. 



61. Growth of Bacteria in Milk. Obtain some absolutely fresh 

 milk. This experiment may be difficult in a city where fresh milk is 

 not easy to obtain. Place one cubic centimeter of the milk in one 

 hundred cubic centimeters of boiled and cooled water, mix thoroughly, 

 and then with a clean sterilized pipette place one cubic centimeter of 

 the diluted milk in each of six test tubes of melted agar culture 

 medium. Mix thoroughly, pour into petri dishes, and set aside for 

 the bacteria to grow. Place the milk at a warm temperature near 

 a radiator for six or eight hours, and repeat the experiment, making 

 six more petri dishes in the same way. Set all aside, and after the 

 bacteria have grown count the number of colonies in each, thus 

 determining the rate of multiplication of bacteria between the first 

 and last experiments. 



