STAINING OF THE SECTIONS. 167 



The sections are placed in water before staining in order 

 that the diffusion of the staining-solution into the tis- 

 sues may be diminished; otherwise our efforts at render- 

 ing the bacteria more conspicuous by decolorizing the 

 tissues in which they are located would rob the bacteria 

 of their color as well. 



The acetic acid and also the alcohol are decolorizers, 

 and are directed toward the excess of stain in the 

 tissues, though they have also some decolorizing action 

 upon the bacteria. The cedar oil and xylol are bodies 

 which mix on the one hand with alcohol, and on the 

 other with balsam. They are known as " clearing 

 fluids/ ' and not only serve to differentiate the compo- 

 nent parts of the tissue, but fill up the gap that would 

 otherwise be left in the process, for a section cannot 

 be mounted in balsam directly from alcohol; the two 

 bodies do not mix perfectly. 



A number of clearing agents are in general use; in 

 fact, almost all the essential oils come under this head. 

 There is one oil of cloves which is very commonly 

 used in histological work; but it must not be employed 

 in tissues containing bacteria. It not only extracts too 

 much color from the bacteria, but causes them to fade 

 after the sections have been mounted for a time. 



When the section thus stained and mounted is exam- 

 ined microscopically it may be found that the tissues 

 still possess so much color that the bacteria are not vis- 

 ible, in which case they have not been decolorized suffi- 

 ciently; or, on the other hand, both bacteria and tissues 

 may have parted with their stains then decolorization 

 has been carried too far. In either case the fault must 

 be remedied in the manipulation of the next section to 

 be mounted. 



