BACTERIOLOGICAL STUDY OF THE SOIL. 463 



scope by the use of a small, sterilized, cylindrical funnel 

 (Fig. 97), the stem of which is bent to an angle of about 

 110 with the long axis of the body. 



The larger part of the aerobioscope is divided into 

 squares to facilitate the counting of the colonies. 



By the employment of this apparatus one can make 

 these analyses at any place, and can, without fear of 

 contamination, carry the tubes to the laboratory, where 

 the cultivation part of the work may be done. 



Aside from this advantage, the filter being soluble, 

 only the insoluble bacteria are left imbedded in the 

 gelatin. 



For general use this method is to be preferred to the 

 others that have been mentioned. 



BACTERIOLOGICAL STUDY OF THE SOIL. Bacterio- 

 logical study of the soil may be made by either breaking 

 up small particles of earth in liquefied media and 

 making plates directly from this, or by what is per- 

 haps a better method, as it gets rid of insoluble particles 

 which may give rise to errors : breaking up the soil in 

 sterilized water and then making plates immediately 

 from the water. 



It must be borne in mind that many of the ground 

 organisms belong to the anaerobic group, so that in 

 these studies this point should be remembered and the 

 methods for the cultivation of such organisms practised 

 in connection with the ordinary methods. It must also 

 be remembered that the nitrifying organisms, everywhere 

 present in the ground, cannot be isolated by the ordinary 

 methods, and will not appear in plates made after either 

 of the above plans. The special devices that have been 

 arranged for their cultivation will be found in the 

 chapter on soil organisms. 



