MICROSCOPICAL EXAMINATION OF BACTERIA 95 



introduction they have served a very useful purpose. Among 

 the advantages to be gained by staining bacteria are: it brings 

 out the minute variations in form and size that would 

 escape unnoticed in the unstained preparation ; it facilitates 

 the differentiation of bacteria from their surroundings, e.g. 

 bacteria from tissue; and in some cases certain stains act as 

 differential agents, aiding in the identification of species. 

 Carmine, from the cardinal insect, was introduced in 1850. 

 Hematoxylin (campeachy, or logwood) was used with great 

 success in histology, but neither of the above stains are appli- 

 cable to bacteria since they give with these forms only a faint 

 tinge. They are chiefly used in bacteriology now as contrast 

 stains. Weigert, in 1877, introduced the anilin dyes. These 

 are derivative of the coal-tar product anilin, C 6 H 5 NH 2 . Ehr- 

 lich first divided these anilin dyes into two groups, according as 

 the staining action depended on the basic or acid portion of the 

 molecule. For example, fuchsin, the acetate of rosanilin 

 (triamidotriphenylcarbinol, C^HigNsCaHA) derives its stain- 

 ing action from rosanilin and is therefore basic. Ammonium 

 picrate derives its staining action from the picric acid part of 

 the molecule and is therefore acid. Basic stains, especially 

 active toward nuclear chroma tin, are the only ones used for the 

 staining of bacteria. The acid stains are especially valuable 

 for the protoplasm of the animal or plant cell. The anilin 

 dyes have all the colors of the rainbow. Some of the most 

 important are : 



