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 Multiplication 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[1] were growing, were transferred 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 | 0 | " |
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 paraffine 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 | 0 | " |
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.
