90 PATHOGENIC BACTERIA. 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 r (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, 1 Dahlia, lodin green. (£) Pararosanilin. The colorless pure para- rosanilin is triamido-triphenyl-karbinol.