802 METHODS OF INVESTIGATING BACTERIA. 



always the most quickly growing organisms which interest 

 us; it is true that by varying the external conditions, 

 more especially the temperature, we may vary the species 

 of organism which grows most quickly in the mixture ; 

 but the method is always uncertain and tedious, more 

 especially as we know very little as regards the most 

 favourable conditions for the growth of the various kinds 

 of organisms. 



Dilution A much better plan is that of diluting the material to 



a great degree. This principle was first recommended 

 by Brefeld, and then by Na'geli and Buchner, and Brefeld 

 employed it more especially for the purpose of inocu- 

 lating the moist chambers described above. According 

 to Brefeld a small quantity of the material is taken and 

 mixed thoroughly with pure sterilised water; the dilution 

 is carried to such a degree that in the quantity contained 

 on the point of a lancet-shaped needle only one germ 

 should be present. When we have convinced ourselves 

 by microscopical examination that this condition has 

 been complied with, we then introduce this amount into 

 each of a series of vessels, and if we employ a large 

 number of vessels, and if in these the same organisms 

 develop which are most numerous in the material em- 

 ployed, the chances are that only one germ was present 

 in each drop. In some of the vessels we will find 

 examples of the other organisms which are present in 

 smaller numbers in the material. If we are isolating 

 mould fungi, the spores of which are difficult to see, it 

 is well to employ a nutrient solution instead of water, in 

 order that the spores may sprout, and thus become 

 larger and more easily visible, and then to dilute the 

 material in the manner above described. 



In the case of the bacteria the microscopical examination 

 is, as a rule, of but little value, because the spores, and 

 even the fully grown organisms, are so small that it is 

 practically impossible to ascertain the presence of a single 

 germ in one drop. In this case we can only get an approxi- 

 mate idea as to the amount of dilution from the micro- 

 scopical appearances. The whole process rests on the 

 assumption that the organisms in which we are interested 



