448 



BACTERIOLOGY. 



Moreover, the buried organism, owing to the filtering power 

 of the soil, is not to be considered as a source of danger, 

 especially if surrounded by dry earth. 



Method of analysis. — The collection of samples of earth 

 from various depths can be readily accomplished by means 

 of Praenkel's earth-borer. For each culture experiment a 

 definite quantity of the soil should be weighed out, or a 

 measured volume taken. The latter is the simpler proce- 

 dure, and can be done with a small platinum spoon which 

 has a capacity of A c.c. 



Fig. 59. Esraarch's apparatus for counting colonies in roll-tubes. >- 



With the above sterile instrument one spoonful of the 

 earth is transferred to a tube of liquid gelatin. The con- 

 tents of the tube are then mixed thoroughly, with a steril- 

 ized platinum wire, and an Esmarch roll-tube is made. The 

 soil and the organisms present are thus brought into per- 

 fect contact with the gelatin, and after a lapse of a few 

 days colonies develop. 



The colonies can be readily counted, either direct, or by the aid 

 of an Esmarch roll-tube counter. This apparatus is shown in Fig. 59. 

 The roll-tube is placed in the holder andjthe average number of 



