MICROORGANISMS OF THE AIR. 187 



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 suspension 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 will sustain them. Winslow 

 has found that a current of seventeen inches per minute is sufficient to 

 sustain B. prodigiosus. The relative humidity of the air is also an im- 

 portant factor. In a supersaturated air solid particles, such as bacteria, 

 become nuclei of condensation for water and quickly settle out. When 

 dust is present in considerable quantities, and certain electrical or mois- 

 ture 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 colonies 

 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 con- 

 ditions of the experiment. Evidently this is of value only for rough 

 comparative work as constantly shifting currents of air usually intro- 

 duce great errors. A somewhat more accurate method is to draw meas- 

 ured 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 

 accurate results still may be obtained by drawing measured volumes of 



