PROBLEM 216 245 



Remove the upper corks from both ends. How long do the 

 candles burn ? 



Remove the lower corks. How long do the candles burn? 



Remove one upper and one lower cork from one end. How 

 long do the candles burn? 



Conclusion. 1. What is the best method of ventilating a 

 room? 



2. Make cross-section sketches and explain the different trials. 

 Use dotted lines and arrows to represent the course of the air. 



Problem 215: To study air for presence of dust. (Home Ex- 

 periment.) 



Materials. Pan, Petri dish with sterile culture medium. 



Method. 1 . Sweep a rug vigorously with a dry broom. Brush 

 your clothes hard after returning from school. 2. Place an un- 

 covered pan of water where a draft from the window will blow over 

 it. 3. Place a sterile culture in Petri dish on window sill for a few 

 moments and then cover; examine after five days. 



Observations. What do you see in the air? What do you 

 notice on the surface of the water? What grows in the dish? 



Conclusion. What is in the air? 



NOTE. A home experiment with culture media to find out dust conditions 

 in different parts of a city would be of much interest for extra credit work. Sug- 

 gested places for exposure of Petri dishes would be (a) a dirty street ; (6) a well- 

 swept and watered street ; (c) a city park ; (d) a city market ; (e) a workshop ; 

 (/) the top of a tall building. Determine by means of colonies formed in the plate 

 the relative numbers of bacteria (and probably the dust content of the air). 



Problem 216 : To determine the best method of cleaning a 

 room. 



Materials. Culture dishes, dry broom, cloth, carpet sweeper, 

 vacuum cleaner. Use four adjoining schoolrooms in which the 

 dust is approximately the same. Expose culture dishes in each 

 room a given length of time, say two minutes, while sweeping. 

 Sweep room 1 with a dry broom, room 2 with a broom over which 

 a wet cloth has been fastened, room 3 with a carpet sweeper, and 

 in 4 use a vacuum cleaner. Place each exposed dish under the 

 same conditions and examine after 2, 3, 4, and 5 days. 



