METHODS OF ISOLATION. 75 



at a temperature favorable to the growth of the bacteria 

 and will remain in its solid condition. 



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 struc- 

 ture, 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 that seen in 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 not only im- 

 possible to pick them out because of their proximity the 

 one to the other, but also because this packing together 

 materially interfered with the production of those char- 

 acters by means of which differences can be seen with 

 the naked eye. The numbers of organisms were then 

 diminished 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 similar portion 

 was added to a third gelatin- bouillon tube, and so on. 

 These were then poured upon large surfaces and allowed 

 to solidify. The results were entirely satisfactory. On 

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

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

 on the plates made from the first dilution they were 

 much fewer in number, but still they were usually too 

 numerous and too closely packed to permit of charac- 



