BACTERIA IN THE AIR AND SOIL 685 



entirely satisfactory. The simple exposure of uncovered gelatin or agar 

 plates for a definite length of time, and subsequent estimation of the 

 colonies upon the plates, yield a result which is variable according to the 

 air-currents and the degree of moisture in the atmosphere, and furnish 

 no volume standard for comparative results. The methods which 

 are in use at the present time depend upon the suction of a definite 

 quantity of air by means of a vacuum-pump through some substance 

 which will catch the bacteria. One of the first devices used for this pur- 

 pose was that of Hesse, who sucked air through a piece of glass tubing, 

 about 70 cm. long and about 3.5 cm. in diameter, the inner surface of 

 which had been coated with gelatin in the manner of an Esmarch roll 

 tube. This method is not efficient, since a large ntunber of the bacteria 

 may pass entirely through the ttibe without settling upon the gelatin. 

 One of the most satisfactory methods at present in use is that in which 

 definite volumes of air are sucked through a sand-filter. Within a 

 small glass tube, a layer of sterilized quartz sand, about 4 cm. in 

 depth, is placed. The sand is kept from being dislodged by a small 

 wire screen. After the air has been sucked through the filter the 

 sand is washed in a definite volume of sterile water or salt solution, 

 and measured fractions of this are planted in agar or gelatin in Petri 

 plates. The colonies which develop are counted. Thus, if two liters 

 of air have been sucked through the filter, and the sand has been 

 washed in 10 c.c. of salt solution, and 1 c.c. of this is planted, with 

 the result of fifteen colonies, then the two liters of air have contained 

 one hundred and fifty bacteria. 



BACTERIA IN SOIL 



Besides the normal bacterial inhabitants of the soU, bacteria reach 

 the soil from the air, in contaminated waters, in the dejecta, excreta, 

 and dead bodies of animals and human beings, and in the substance of 

 decaying plants. It is self-evident, therefore, that the distribution of 

 bacteria in soil depends largely upon the density of population and the 

 use of the soil for agricultural or other purposes. Thus, bacteria are most 

 plentiful in the neighborhood of cess-pools or in manured fields and gar- 

 dens. Such conditions, however, may be regarded as abnormal. Even 

 in uncultivated fields there is a constant bacterial flora in the soil which 

 is of great importance in its participation in the nitrogen cycle, a pha,se 

 of the bacteriology of soil which has been discussed in detail in another 



