188 BACTERIOLOGY. 



Each step in the above process has its definite object. 

 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. 

 They are employed to reduce the excess of stain in the 

 tissues, though they also have 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 essen- 

 tial 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 for tissues contain- 

 ing 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 

 l)ossess so much color that the bacteria are not visible, in 

 which case they have not been decolorized sufficiently ; 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. 



