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 ; (&) a well- 

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

 (J) 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. 



