METHODS OF ISOLATION. 73 



If, however, it is possible to discover some substance 

 which possesses the property of being at one time fluid 

 and at another time solid, which can be added to this 

 bouillon without in any way interfering with the life- 

 functions of the bacteria, then, as solidification sets in, 

 the organisms will be fixed in their positions and the 

 conditions will be analogous to that seen on the bit of 

 potato. 



Gelatin possesses this property. At a temperature 

 which does not interfere with the life of the organisms 

 it is quite fluid, whereas when subjected to a lower tem- 

 perature it solidifies. When once solid it may be kept 

 at a temperature favorable to the growth of the bacteria 

 and will retain 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. It was 

 necessary, however, to use a more dilute mixture of bac- 

 teria 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 impossible 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 characters by means of 

 which differences can be seen with the naked eye. The 

 numbers of organisms were then diminished by a process 



