DUST AND ITS DANGERS. 



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 instead 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. 



