PROBLKJM 21(; 215 



Remove the upper corks from holli ends. II..\v lonu .l<> flie 

 candles burn ? 



Remove the lower corks, jlow long do tlic candles burn? 



Remove one upper and one lower coik fnun 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 difTerent trials. 

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



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

 periment.) 



Materials. — Pan, Petri dish with sterile culture medium. 



Method. — 1. Sweep a rug vigorously with a dry l)room. Bru.-<h 

 your clothes hard after returning from school. 2. Phice 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 tlish? 



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 he (a) a dirty street ; ('<) a wi-U- 

 swept and watered street ; (c) a city park ; {d) a city market ; (f ) a work.sljop ; 

 (/) 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 determhie the best viethod of elediiin^ a 

 room. 



Materials. — Culture dishes, dry broom, cloth, carpc^t 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 mimites, while sweeping. 

 Sweep room 1 with a dry broom, room 2 with a broom ov(^r which 

 a wet cloth has been fastened, room 3 with a carpet sweeju'r, and 

 in 4 use a vacuum cleanei-. Place each exj)osed dish uniler the 

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



