PRINCIPLES IN METHODS OF ISOLATION 107 



in their positions, and the conditions would be analogous 

 to those seen on the bits of potato, bread or leather. 



Gelatin possesses this property, and it was, therefore, 

 used. At a temperature which does not interfere with the 

 life of the bacteria it is quite fluid, whereas when subjected 

 to a lower temperature it solidifies. When once solid it 

 may be kept at a temperature favorable to the growth of 

 the bacteria and will remain in its solid state. 



Gelatin was added to the fluids containing mixtures of 

 bacteria, and the whole was then poured upon a large, flat 

 surface, allowed to solidify, and the results noted. It was 

 found that the conditions seen on the slice of moldy potato 

 could be reproduced; that the indivduals in the mixture 

 of bacteria grew well in the gelatin, and, as on the potato, 

 grew in colonies of typical macroscopic peculiarities, so 

 that they could easily be distinguished the one from the 

 other by their naked-eye appearances. (See Fig. 14.) It 

 was necessary, however, to use a more dilute mixture of 

 bacteria than the original decomposed bouillon. The 

 number of individuals in the tube was so enormous that on 

 the gelatin plate they were so closely packed together that 

 it was impossible to pick them out, not only because of 

 their proximity the one to the other, but also because this 

 packing together materially interfered with the production 

 of those characteristic differences visible to the naked eye. 

 The numbers of the organisms were then diminished by a 

 process of dilution, consisting of transferring a small portion 

 of the original mixture into a second tube of sterilized bouillon 

 to which gelatin had been added and liquefied; from this 

 a portion was added to a third gelatin-bouillon tube, and 

 so on. These were then poured upon large, cold surfaces 

 and allowed to solidify. The result was entirely satisfactory. 



