EXAMINATION OF AIR, SOIL, AND WATER 227 



The Examination of the Soil. The upper layers of the 

 soil contain a great many bacteria, but because of the difficulty 

 in analyzing the same, the results are neither accurate nor con- 

 stant. The principal trouble lies in the mixing of the earth 

 with the nutrient medium; little particles of ground will cling 

 to the walls of the tube, or be embedded in the gelatin, and 

 may contain within them myriads of bacteria. As with water, 

 the soil must be examined immediately or very soon after it is 

 collected, the bacteria rapidly multiplying in it. 



When the deeper layers are to be examined, some precautions 

 must be taken to avoid contamination with the other portions of 

 the soil. One method, very laborious and not often practical, 

 is to dig a hole near the spot to be examined and take the earth 

 from the sides of this excavation. 



Frankel's Borer. Frankel has devised a small apparatus 

 in the form of a borer, which contains near its lower end a small 

 cavity, which can be closed up by turning the handle, or opened 

 by turning in the opposite direction. 



It is introduced with the cavity closed, and when it is at the 

 desired depth, the handle is turned, the earth enters the cavity, 

 the handle again turned, incloses it completely, and the borer is 

 then withdrawn. 



The earth can then be mixed with the gelatin in a tube, and 

 this gelatin then rolled on the walls of the tube after the man- 

 ner of Esmarch, or it can be poured upon a glass plate, and 

 the colonies developed so. 



Another method is to wash the earth with sterilized water, 

 and the water then mixed with the gelatin, as many of the 

 germs are taken up by the water. 



The roll-cultures of Esmarch give the best results, many of 

 the varieties usually found being anaerobic. 



Animals inoculated with the soil around Berlin die almost 

 always of malignant edema, and with that of some other towns 

 invariably of tetanus. Many of the germs found are nitrogen 

 formers and play a great role in the economy of the soil. 



Nitrifying organisms are found in the superficial layers of the 

 earth. Organic matters found in sewage and in the fecal 



