METHODS OF OBSERVING BACTERIA. 77 



mixed up in a drop of sterile bouillon or water and ex- 

 amined. 



In the early days of study efforts were made to facili- 

 tate the observation of bacteria by the use of carmin and 

 hematoxylon. Both of these reagents tinge the proto- 

 plasm of the organisms a little, but so unsatisfactorily 

 that since Weigert introduced the anilin dyes for the 

 purpose both of these tissue-stains have been rejected. 

 The affinity between the bacteria and the anilin dyes is 

 peculiar, and many times is so certain a reaction as to 

 become an essential factor in the differentiation of 

 species. 



For the study of bacteria in the stained condition we 

 now employ the anilin dyes only. These wonderful 

 colors, as numerous as the rainbow hues, are coal-tar 

 products. Hiippe classifies them as follows : 



A. Dyes prepared from anilin oil. 



1. Oxidation-products of pure anilin : 



Methylene blue, 



Chlorhydrin blue (basic indulin). 



2. Oxidation-products of pure toluol : 



Safranin. 



3. Oxidation-products of mixed anilin and toluol : 



(a) Rosanilin. When pure this is triamido- 



diphenyl-toluyl-karbinol. 



Fuchsin rosanilin hydrochlorate. It is 

 often mixed with the acetate and the 

 pararosanilin acetate and hydrochlo- 

 rate. The pure rosanilin hydrochlorate 

 should always be chosen for purposes of 

 staining. 



Azalein is rosanilin nitrite. 



Methylized and ethylized rosanilin : 

 lodin violet, 

 Dahlia, 

 lodin green. 



(b) Pararosanilin. The colorless pure para- 



rosanilin is triamido-triphenyl-karbinol. 



