128 ESSAYS ON BACTERIOLOGY. 



-ward, over a flame till the solution steams, reveals but 

 few varieties unstained. Again we stain a similar 

 series, using a cold solution containing 5 per cent, of 

 carbolic acid, repeating the procedure with the same 

 solution heated, and with solutions, cold and hot, con- 

 taining a minute quantity of potassium hydrate. In 

 the last specimens, stained with the mordants, we shall 

 probably find all the different varietis to have taken 

 up the dye. 



If now we subject all of these specimens for a few 

 seconds to a 25 per cent, solution of nitric acid, the 

 color, as seen by the unaided eye, fades from all. 

 Microscopic examination shows that almost all of the 

 bacteria have been decolorized by the acid. One or 

 two varieties only retain the stain, standing out con- 

 spicuously colored upon a colorless background. 

 Upon close study these will probably be found to be 

 just the ones which resisted the simpler staining, re- 

 quiring the most energetic dyes. The tubercle bacil- 

 lus, for instance, stains with difficulty, but, once col- 

 ored, it resists the decolorizing agent to which most 

 others succumb. Selecting such a specimen, we stain 

 it again, after thorough washing to remove the acid, 

 with a watery solution of a contrast color. The re- 

 sistant bacteria do not take the dye, while other bac- 

 teria, mucus, and debris, making up the field, do. 

 The former present the one color, the latter the other. 

 And thus we see illustrated the selective action of the 

 ■dyes, and the staining and decolorizing reaction of 



