EXAMINATION OF LIVING BACTERIA. 19 



Generally quite different ones make their appearance 

 when finely pounded meat is mixed with ditch- water^ 

 and left to decompose. 



In all these ways. a mixture of bacteria is obtained ; 

 that is to say, bacteria of very different forms and 

 kinds are found mixed together. It isj moreover^ 

 impossible to say beforehand which forms will be de- 

 veloped, for this depends on the kind of germs which 

 happen to be present, and also on the varying chemi- 

 cal constitution of the special nutrient fluids prepared 

 as above. 



By the following method, a pure culture — that is to 

 say, one in which only one kind of bacterium develops 

 itself — may be obtained. Hay is left in water for 

 twenty -four hours ; the resultant liquid is then strained 

 thi-ough a linen cloth, and diluted, until a volume, ten 

 centimetres in depth, is, of a light chestnut-brown 

 colour. 



The success of this experiment depends upon the 

 fact that the spores of the hay-bacillus [Bacillus sub- 

 tilis Ehrenberg) can endure the heat of boiling water 

 for an hour without being killed. The liquid is poured 

 into a Florence flask, until the latter is three-quarters 

 full. The mouth of the flask is then stopped up with 

 a cotton-wool plug, and its contents are kept simmer- 

 ing gently — only a small amount of steam being 

 formed, — for an hour. In this manner all other organ- 

 isms and their germs are killed, the spores of the hay- 



