156 BACTERIOLOGY 



structed and kept clean, deliver water which is poor in 

 germs, or entirely free from them (C. Frankel). 



•According to Kirchner, the freedom of ground-water 

 from germs is due to the filtering action of the soil, and 

 therefore, where this is too coarse and porous, the filtering 

 power fails, and the whole or a part of the germs met with 

 in the upper stratum of earth pass unhindered into the 

 ground-water. According to Eeimers, the germs contained 

 in the deeper part grow more slowly than those derived 

 from superficial layers. 



Method of examination. — If it is desired to examine soil, 

 or the dust of windows and rooms, for micro-organisms, a 

 small sample is taken — freshly, if possible — and introduced 

 into sterilised nutrient gelatine, which has been melted but 

 is not too hot, in order to prepare a roll-culture by Von 

 Esmarch's method. Cultures of this form are preferable 

 to plates, because the small particles of earth do not sink 

 to the bottom of the tube and get missed, as is liable to 

 happen in pouring out the contents on the plate. Special 

 instruments are employed for obtaming earth from different 

 depths, of which a borer constructed by C. Frankel is that 

 principally in use. 



When the individual islets have been isolated by means 

 of the roll-culture, they are transferred to different plates, 

 in order to obtain j)ure cultivations of the particular 

 organisms. The examination of anaerobic micro-organisms 

 is carried on by the methods detailed above. 



The brown mould is very widespread in the earth as 

 well as in air (p. 104), and some micrococci are found which 

 liquefy gelatine. Generally speaking, the cocci are more 

 numerous than the bacilli. 



Bacillus mycoides (earth bacillus). — Pliigge found, as a 

 very frequent guest in the soil of fields and gardens, a 

 micro-organism whose rods strongly resemble those of 



