DUST AND ITS DANGERS. 15 



easily handled and is equally efficient as a 

 filter. We plant the sand together with the 

 dust which it has caught in the melted culture- 

 medium, allow it to cool and then stand for a 

 few days, and when the colonies are grown they 

 are easily distinguished from the sand particles 

 by their shape, color, etc., and can be readily 

 counted. Or, we may use granulated sugar 

 for a filter, which finally dissolves in the culture- 

 medium, leaving the bacteria to grow in due 

 time. This may be called the " filtration 

 method " of air analysis. 



As it requires an accurate and somewhat 

 complex and cumbersome apparatus to force 

 or draw the air through either the cotton or 

 sand filter, another and simpler method is 

 often resorted to, which, though in some re- 

 spects less accurate, still gives very useful 

 results when we wish simply to compare the 

 germ ingredients of the air in one place with 

 those in another under similar general condi- 

 tions. 



This simpler method consists in pouring into 

 a series of perfectly clean shallow glass dishes 

 a thin layer of the warm gelatinous culture- 



