TEXT-BOOK OF BACTERIOLOGY. 351 



2. INVESTIGATION OF THE SOIL. 



The method of bacteriological examination of the soil is much 

 less complete than that of the air. 



For the purpose of obtaining- comparable results, weighed or 

 measured quantities of earth are brought into more or less intimate 

 contact with gelatin. The sample is distributed as uniformly as 

 possible by means of a sterilized scalpel over the surface of a plate 

 supplied with the medium. But all the germs are not developed 

 nor can they produce independent colonies. The particles of earth 

 lie but loosely on the medium and retain numerous micro-organ- 

 isms in their interior, thus depriving the result of all its value. 



Some try to mix the sample of soil directly with the gelatin by 

 pouring it into the test-tube before the contents of the latter are 

 poured on the plate. But a large part of the material cannot be 

 prevented from remaining behind in the tube and thus being lost 

 for examination. Even if this defect is remedied as far as possible, 

 by preserving the emptied test-tube and taking into consideration 

 the colonies developed in it, no certain results have been obtained. 



The number of germs in the upper strata of the earth is usually 

 extremely great. 



Others have washed the sample of soil for hours with sterilized 

 distilled water before placing measured quantities of it on gelatin ; 

 but this investigation is complicated and clumsy, and there is no 

 absolute certainty of having actually loosened and washed out all 

 the germs from their substratum. 



Relatively the best, though by no means perfect procedure, is 

 this : pour the sample of soil directly into the liquid gelatin in the 

 test-tube, rub and stir it thoroughly by means of a strong plati- 

 num needle and then distribute it, according to Esmarch's method, 

 on the walls of the tube. All germs approximately will thus be 

 developed and comparable results will be obtained. 



The most difficult, and at the same time most important part 

 of the whole procedure, is to procure available, reliable material. 

 Superficial strata of soil can be taken up without difficulty ; but 

 the more we penetrate into the earth, the more it will be felt 

 that samples can hardly be obtained which are without any doubt 

 derived from the locality needed. 



For this procedure it will be safest if we are able to dig soil from 

 suitable depths and to utilize portions on the sides of the excava- 

 tion. Such a favorable opportunity is but rarely offered, however, 

 and investigation will, as a rule, surely be discouraged under such 

 circumstances. The use of common boring tools does not prevent 



