EXAMINATION OP AIH. 449 



colonies in a square cm. is ascertained in the same manner as de- 

 scribed under water analysis. The area of the gelatin cylinder is 

 then calculated from the formula t D a; in which n is 3.1416, D the 

 diameter of the tube and a the length of the gelatin cylinder. The 

 total number of colonies can then be determined. The kind of organ- 

 isms present — bacteria, moulds, etc. — can be determined by a study 

 of the colonies, and by further culture and examination. 



In this way it is easy to determine approximately the 

 number and kind of organisms present. Unfortunately, 

 this method is not adapted for the detection of anaerobic 

 bacteria, which are apparently widely distributed in the 

 earth, and are represented by the well-known bacilli of 

 tetanus, malignant edema and symptomatic anthrax. 

 These have thus far been obtained from the soil only by 

 the indirect method mentioned on p. 446. 



Air. 



The bacteria present in the air are derived almost \en- 

 tirely from the soil. When the surface of the earth be- 

 comes dry, the fine particles of dust are readily taken up 

 by gusts of wind and may then be carried upward to a con- 

 siderable altitude. To a slight extent, germs may enter 

 the air from the water. This can happen, however, only 

 when the water is violently thrown into a spray as in the 

 neighborhood of a water-fall or in the case of storm-lashed 

 waves. The extremely fine particles of water are carried 

 into the air and eventually may dry up leaving the solid 

 organisms behind. 



It should be clearly understood that germs do not and 

 cannot enter the air of themselves. They must always be 

 torn by force from the soil or from the water. Moreover, 

 what is equally important, they are not removed from moist 

 surfaces or from water by ordinary currents of air. The 

 most pathogenic organism growing on the ordinary culture 



