48 



Bactrria in Relation to Countrij Life 



ticular sample of air, or, at least, none that would grow 

 under those special conditions. 



Value of Pasteur's method. — This method of Pasteur 

 merely showed whether there were any microorganisms 

 at all in a given volume of air. It did not show how many 

 there were in it, since the resulting growth could have 

 been due to one or to several organisms. Other methods 



Fig. 15. Liquefied agar ready for inoculation and pouring into sterile Petri dishes. 



have been employed, therefore, to show, not only the 

 mere presence, but, also, the number and kind of bac- 

 teria in a given quantity of air. 



Quantitative methods. — The methods employed for 

 this purpose consist in passing measured quantities of 

 the air to be examined through vessels or tubes, whose 

 walls are lined with nutrient gelatin, or through small 

 quantities of solid materials, or of liquid. In the former 

 case, the microorganisms of the air gradually fall on 



