Page:Popular Science Monthly Volume 66.djvu/478

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YGIENE is the one secondary school subject that every pupil will find necessary throughout life. In few high schools, however, has it received the treatment a subject of such importance demands. This article is a plea for definite, forceful experiments and demonstrations in hygiene comparable with the experiments and demonstrations so long deemed essential in physics and chemistry. The following experiments could be done in any secondary school.

1. To demonstrate Bacteria in the Air.—A test-tube of sterile nutrient agar was melted by immersing it in boiling water. The agar was then poured into a sterile Petri dish, where it cooled and solidified in a thin film. The cover of the dish was then removed and the medium exposed to the air for ten minutes. The cover was replaced and the dish set aside. Two days later, there had developed on the agar 63 colonies of bacteria. A dish prepared in the same way, but not exposed to the air, developed no colonies.

2. To demonstrate Bacteria in Water.—A test-tube of agar was melted and placed in water at 42° C, a temperature slightly above that of the human body. At this temperature, agar remains liquefied, but it is not hot enough to kill bacteria. Two tiny drops of drinking water were transferred to this melted agar by means of a small, sterile loop of platinum wire. The agar was then poured into a sterile Petri dish. A few days later, the dish was found to contain fourteen colonies of bacteria.

3. To demonstrate Bacteria in Milk.—In the same way, a test-tube of agar was inoculated with two tiny drops of milk, and the agar poured into a sterile Petri dish. In this dish 192 colonies developed.

4. To show the Effect of Heat on Bacteria.—Three test-tubes of milk were taken. The first was kept at room temperature, the second was immersed in water at 60° C, and the third placed in boiling water. At the end of twenty minutes, Petri dishes were prepared from each tube, with the following results:

5. To determine the Effect of Freezing upon Bacteria.—Two test tubes of water were used. One was kept at room temperature, the