1012 BACTERIA IN AIR, SOIL, WATER, AND MILK 



The bacteria thus discharged arc then subject to the process of 

 drying and often are exposed to direct sunlight for a considerable 

 period before they are again taken up in the air. 



The methods of estimating the bacterial contents of the air are 

 not 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 purpose was that of Hesse, who sucked 

 air through a piece of glass tubing, about 78 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 number of the bacteria may pass entirely 

 through the tube 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 soil, 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 



