70 BACTERIOLOGY. 



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 

 |)ptato. 



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 

 temperature it solidifies. When once solid it may be 

 kept at a temperature favorable to the growth of the 

 bacteria and 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 structure, 

 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 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 impossible to pick them 

 out because of their proximity the one to the other, 

 bul also because this packing togetlier materially inter- 

 fered 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 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 similar portion was added to a 



