PRACTICAL BACTERIOLOGY. 



useful specific characteristiCj by means of which these 

 species can be distinguished from others, in which 

 such streaks never occur, except in quite old culti- 

 vations. 



It may be farther observed that some species stain 

 easily and intensely ; whilst others of the same ag6 

 only become coloured very slowly, and even if they are 

 left for a long time in the staining solution, never be- 

 come intensely red or violet. This is partly caused by 

 the fact that some kinds grow old earlier than others, 

 and hence with these much younger cultures than 

 usual must be used; and partly that some bacteria 

 absorb staining reagents much more readily than 

 others. There are bacteria which only become imper- 

 fectly coloured even when they remain for days in the 

 staining solution. These differences in their behaviour 

 towards staining reagents are of great use in differen- 

 tiating various species. Great variety is also shown 

 in the persistency with which different kinds retain 

 their colour when once stained. 



If we make such stained cover-glass preparations of 

 a number of species with material obtained from our 

 pure cultures, we shall very soon notice how differ- 

 ently various species behave towards decolourising 

 reagents. First, let us try the effect of a very weak 

 decolouriser, ten per cent, alcohol. In half a miiiute 

 it is seen that nearly all the species have parted with a 

 large proportion of the staining material that they had 



