METHODS OF ISOLATION. 95 



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 

 potato could be reproduced ; that the individuals in the 

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

 the potato, grew in colonies of typical macroscopic pecu- 

 liarities, 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 decom- 

 posed 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 dimin- 

 ished by a process of dilution, consisting of trans- 

 ferring 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. 

 On the gelatin plates from the original tube, as was ex- 

 pected, the colonies were too numerous to be of use; 

 on the plates made from the first dilution they were 

 much fewer in number, but usually they were still too 



