96 BACTERIOLOGY 



CHAPTER V 



THE BACTERIOLOGICAL ANALYSIS OF AIR 



Micro-organisms in the air. — Floating in the air are 

 particles of dust consisting of organic substances, amongst 

 which are also to be included, as a rule, dried-up colonies 

 of micro-organisms. Such may either sink downwards 

 of themselves under the influence of gravity, and so be 

 caught, or they may be obtained by calling in the aid of 

 currents of air, but in all cases they must be transmitted 

 to a suitable nutrient medium before they can develop. 

 As a rule we find in the air moulds, yeasts, and the spores 

 of bacteria. On the open sea, far out from shore, the 

 number of micro-organisms is considerably smaller, and in 

 like manner the air on high mountains is almost entirely 

 free from germs, or at least there are but few, whereas on 

 the plains 100 to 500 germs capable of living have been 

 counted in each cubic centimeter. The air of dwelling- 

 rooms contains them in considerable numbers only when 

 they have been whirled up from between the flooring and 

 from the coatings of the walls, and this detachment of 

 bacteria by draughts of air can only take place when the 

 surfaces are dry. 



Simple methods of examining air. — The simplest way of 

 examining air consists in letting a plate prepared with agar 

 or gelatine stand in any locality for a definite time, and 

 afterwards placing it in a moist chamber, when colonies of 

 micro-organisms will form in a few days. Agar plates 



