DUST AND ITS DANGERS. 1 9 



more accurate and cumbersome apparatus dif- 

 ficult or impracticable. We can go about with 

 our innocent-looking little case of glass boxes, 

 partly filled with nutrient gelatin, as does the 

 amateur photographer with his detective cam- 

 era ; though insteadr of " pulling the string, 

 touching the button, and leaving the rest to 

 the manufacturer," we raise the cover, take the 

 time* and let Nature do the rest. 



We are now ready to look at the results of 

 a series of so-called biological analyses of the 

 air of various places. We mean by biological 

 analysis of air, in distinction from the chemical, 

 an analysis which has for its object the deter- 

 mination of the number or character, or both, 

 of the living germs, or micro-organisms which 

 may be suspended in it. 



