DUST AND ITS DANGERS 



and its Dangers." The dangers, as he pointed out, are 

 not in the dust itself, but in the possible presence in the 

 dust of many tiny living organisms known as bacteria, 

 yeasts, or molds. It is very easy to prove the presence of 

 these living plants, for such they are, in the air of the school- 

 room. Let us obtain some previously prepared material 

 known as cul- 

 ture medium. 1 

 This has been 

 placed in a Petri 

 dish, a small 

 glass dish with 

 a loosely fitting 

 glass cover. The 

 dish and con- 

 tents have been 

 t h o r o ug h ly 

 sterilized, or 

 rendered free 

 from all living 



* Plate culture exposed for five minutes in a school hall 



of h P a tin & where pupils were passing to recitations. Each spot is 



o ' a colony of bacteria or mold. 



Thus we know 



that the culture medium and everything inside of the dish is 

 dead. If this dish is now opened for five minutes, exposing 

 the surface of the culture medium to the air of the room, then 

 closed and put away in a warm dark place for two or three 

 days, a number of little dots of different sizes and colors will 

 be found growing on the surface of the culture medium (see 



1 This culture medium may be bought already prepared. It is a gela- 

 tine or agar-agar preparation upon which germs grow readily. It has been 

 previously rendered free from all living things by sterilization. 



