56 BACTERIOLOGY. 



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 separated 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, but 

 also because this close packing together 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 pro- 

 cess 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 third galatin- 

 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 expected, 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 characteristic growth ; but on the 



