392 CLINICAL BACTERIOLOGY. 



up to the present, been found in the earth only when but a 

 short time has elapsed after the bacteria in question have 

 been introduced at the given point with the disease-pro- 

 ducts (cholera or typhoid dejections, etc.). Typhoid-bacilli 

 deposited at a depth of y 2 meter retain their capability of 

 development, under favorable conditions, for five and a half 

 months, and perhaps still longer. 



On the whole, it may be stated that pathogenic germs 

 may multiply upon and within the surface of the earth if 

 the atmospheric temperature be high ; but that, as a rule, 

 they are suppressed by the competition of saprophytes. 

 In the deeper layer the conditions for their propagation 

 are much less favorable. 



The burial of the bodies of individuals that have died 

 from infectious diseases is scarcely capable of giving rise to 

 infection. The accompanying bacteria are overrun by the 

 saprophytes, and even if a small number as, for instance, of 

 tubercle-bacilli may persist for months, perhaps for years, 

 the opportunity is, on the whole, seldom afforded for their 

 being carried from a depth to the surface of the earth or 

 into the subsoil water. 



AIR. 



Method of Examination. /. Procedure of Hesse. A 

 glass tube 70 cm. long and 3.5 cm. in diameter is sterilized 

 and coated with gelatin, which, in the same way as in 

 Esmarch's tubes, is distributed uniformly upon the inner 

 surface by rotation in cool water. By means of an aspirator 

 air is then drawn 'through this tube at the rate of a liter in 

 from two to four minutes. At this slow rate of speed the 

 germs of the air are deposited upon the gelatin, in which 

 they subsequently develop into colonies. This method 

 permits of examination of only relatively small quantities 

 of air (from 10 to 20 liters). 



2. Procedure of Petri. A sand-filter, 3 cm. thick, sup- 

 ported upon two wire nets, is fastened in a short, glass tube, 

 with a diameter of from 1.5 to 2 cm. ; the whole is steril- 

 ized, and air is permitted to stream through in a rapid cur- 

 rent. The sand, which should have a grain of from y^ to 

 y. mm. in size, filters with certainty all of the germs con- 

 tained in the air. After from 50 to 100 liters of air have 

 been drawn through the apparatus, the entire filter is intro- 



