5« PRACTICAL BACTERIOLOGY. 



number of good specific characteristics are famished 

 liy the way in which the difierent species cause the 

 gelatine to liquefy ; these are however only fully 

 recognised by degrees, even by the practised eye. 

 Some only liquefy the gelatine just so far as the colony 

 extends, others form only insignificant point-like 

 colonies, which are surrounded by a ring of fluid 

 gelatine often twenty times as great in diameter ; and 

 betweeu these extremes there are all imaginable in- 

 termediate cases. Many turn the gelatine into a 

 tenacious sticky substance, others into a light mobile 

 fluid, hardly to be distinguished from bouillon. Some 

 colonies {Bacillus proteus) liquefy the gelatine only oa 

 the surface and then spread themselves out in it. Of 

 course the rapidity with which the gelatine is liquefied 

 depends very much upon the temperature and espe- 

 cially upon the strength of the preparation ; it is more 

 difiicult to liquefy one containing ten per cent, of gela- 

 tine than one containing only five per cent. It is 

 therefore necessary in order to compare results ob- 

 tained to work with media of the same strength. 



In cultivations of agar-agar these characteristics 

 disappear, as no bacteria have the power of liquefying 

 it. 



The different colours of the colonies also afford dis- 

 tinguishing traits. The colonies are only very rarely 

 colourless and transparent ; as a rule they are more 

 or less intensely coloured ; white, yellow, red, blue. 



