474 POPULAR SCIENCE MONTHLY. 



SIMPLE BACTEEIOLOGY FOR PUBLIC SCHOOLS. 



By LILLIAN CHAPIN, B. B., 



INSTRUCTOR IN CALUMET HIGH SCHOOL, CHICAGO. 



"1 T YGIENE is the one secondary school subject that every pupil 

 -■ — ■- will find necessary throughout life. In few high schools, how- 

 ever, has it received the treatment a subject of such importance de- 

 mands. 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 ex- 

 posed 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 col- 

 onies 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 G0° 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 : 



Unheated milk 192 colonies. 



Milk heated to 60° C 13 



Milk heated to 100° C 5 



5. To determine the Effect of Freezing upon Bacteria. — Two test- 

 tubes of water were used. One was kept at room temperature, the 



