CULTURE METHODS AND APPARATUS 87 



bearing bacilli by heating the culture before incubating it to 

 nearly or quite the boiling point of water. By this method 

 it is possible to isolate in pure culture such bacteria as the 

 hay bacillus and the potato bacillus. Gayon and Dupetit 

 devised a culture tube to take advantage of the fact that 

 certain bacteria were much more motile than others. These 

 tubes were provided with a long glass spiral through which 

 the bacteria were obliged to travel. The most motile forms 

 would get through the tube first and could be removed before 

 the slower forms arrived. Parietti used a disinfectant to 

 inhibit the growth of certain bacteria without interfering 

 with the growth of others, and by means of a weak solution 

 of carbolic acid and hydrochloric acid in water he found it 

 possible to separate B. coli from other bacteria, and he even 

 claimed that he could separate this bacillus from B. typhosus. 

 These methods are occasionally used at the present time, but 

 the difficulties are the same as those indicated under the 

 head of Fractional Methods, i.e. it is necessary to start from 

 an unknown mixture, and it is, therefore, impossible to know 

 the results. The weaker species may not be destroyed, so 

 that after the stronger species begin to degenerate the others 

 get a chance to multiply ; and then, again, two or more species 

 may thrive equally well. 



The Dilution Methods. In these methods the principle 

 involved is to dilute material with a sterile liquid, as water, 

 to such an extent that a single unit of the mixture contains 



not more than one bacterial cell. Lister, in 1878, obtained a 



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