SIMPLE BACTERIOLOGY FOR PUBLIC SCHOOLS. 475 



other was frozen solid by means of a mixture of salt and ice. At the 

 end of twenty minutes the frozen specimen was allowed to melt. 

 Petri dishes were then prepared from each, with the following results: 



Not frozen 272 colonies. 



Frozen 294 " 



In a similar experiment with milk, 192 colonies developed from the 

 sample kept at room temperature, and 193 colonies from the sample 

 frozen. 



6. To show the Effect of Temperature on the Rate of Multiplica- 

 tion of Bacteria. — Three test-tubes of milk were used. A Petri dish 

 made from the milk at this time showed 80 colonies of bacteria. The 

 first tube was placed in an ice chest, the second was kept in a locker, and 

 the third was put in an incubator. Petri dishes made from each, 

 twenty-four hours later, showed the following counts: 



Ice chest (4° C.) 70 colonies. 



Locker (20° C.) 30,000 



Incubator (37° C.) 100,000 



7. To show the Effect of Drying on Bacteria. — Three loops full 

 of beef broth in which typhoid fever germs* were growing, were trans- 

 ferred to each of two dry, sterile test-tubes. Into the first tube, melted 

 agar was poured, and then poured out into a sterile Petri dish. The 

 drops in the second test-tube were allowed to dry, and a day or two 

 later a Petri dish was prepared from this tube in the same way. The 

 two Petri dishes showed the following counts: 



Specimen not dried 6,000 colonies. 



Dried specimen " 



8. To determine the Effect of Sunlight on Bacteria. — Two slips 

 of filter paper were wet with a beef-broth culture of typhoid. The 

 papers were placed in sterile test-tubes, which were sealed with paraf- 

 fine to prevent drying. One of the test-tubes was placed in a locker. 

 The other was exposed to sunlight for three hours. Petri dishes were 

 then prepared, which showed the following counts: 



Paper not exposed to sunlight 100,000 colonies. 



Paper exposed to sunlight " 



9. A study of Antiseptics. — Four test-tubes were prepared, each 

 containing 5 c.c. of water to which typhoid bacilli had been added. 

 To the first tube nothing was added. Half a cubic centimeter of a 

 weak solution of the antiseptic to be tested was added to the second. 

 To the third, 1 c.c, and to the fourth 2 c.c. of the antiseptic. Twenty 



* In repeating this experiment in high schools, harmless bacteria must, of 

 course, be used. The colon bacillus is recommended for this purpose. 



