MICROORGANISMS OF THE AIR 249 



grooming animals, sweeping a floor or carpet will multiply the dust and 

 bacterial content of the air many times. In a similar manner, tiny, 

 germ-holding droplets may be scattered by the splashing of sewage or of 

 fermenting or putrefying liquids, and in speaking, sneezing or coughing. 



Conditions for Subsidence of Bacteria. — The length of time 

 during which an organism may remain suspended in the air is dependent 

 upon several factors. Small particles settle out more slowly than large 

 for the reason that as the size of an object is decreased, the surface area 

 decreases less rapidly proportionately than the volume. The lifting 

 effect of air currents depends upon the ratio of surface area to volume 

 and specific gravity. The smaller the object, therefore, the greater is 

 the resistance to subsidence. Consequently, bacteria usually settle 

 out of air very slowly if free in a quiet atmosphere. The time of sus- 

 pension is determined also by the velocity of the air currents. While 

 considerable velocity may be necessary to dislodge microorganisms and 

 bring them into suspension, a very slight air current wiU sustain 

 them. Winslow has found that a current of 17 inches per minute is 

 suflEicient to sustain B. prodigiosus. The relative humidity of the air is 

 also an important factor. In a supersaturated air solid particles, such 

 as bacteria, become foci of condensation for water and quickly settle 

 out. When dust is present in considerable quantities, and certain elec- 

 trical or moisture conditions exist, flocculation occurs and the larger 

 bodies so formed subside rapidly. The character and abundance of 

 surfaces with which the suspended particles may come in contact also 

 play an important part. Moist surfaces are much more effective in 

 retaining particles than those which are dry. 



Determination of the Number of Bacteria in the Air. — The 

 number of bacteria in the air is frequently determined by exposing open 

 petri dishes of gelatin or agar in different places for definite periods. 

 This is a comparative quantitative method only. The number of colo- 

 nies developing upon these plates will give the number of dust particles 

 having living spores or cells upon them that fall in the given area under 

 the conditions pf the experiment. Evidently this is of value only for 

 rough comparative work as constantly shifting currents of air usually 

 introduce great errors. A somewhat more accurate method "is to draw 

 measured volumes of air into a flask, the bottom of which is covered 

 with a layer of gelatin or agar. The colonies which develop represent 

 the number of organisms which settle out from the given volume. More 



