160 BACTERIOLOGY. 



is much greater and more rapid than when the same 

 bacteria are located in the interstices of tissues. These 

 differences are not in the bacteria themselves, but in the 

 obstruction to diffusion offered by the tissues in which 

 they are located. 



The result of absence of diffusion may easily be illus- 

 trated. Prepare a cover-slip preparation, dry it care- 

 fully, fix it, and, without allowing water to get on it 

 from any source, attempt to stain it with a solution of 

 the dyes in absolute alcohol, washing it out subsequently 

 with absolute alcohol; the result is negative. The abso- 

 lute alcohol does not possess the property of diffusing 

 into the dried tissues, and hence, as has been stated 

 before, alcoholic solutions of the stain ing-dyes should 

 not be employed. The staining-dyes should always be 

 watery.^ 



Decolorizing-solutions. — As regards the employ- 

 ment of decolorizing-agents, it must always be borne in 

 mind that objects which are easily stained are also easily 

 decolorized, and those that can be caused to take up the 

 stain ing-material only with difficulty are also very diffi- 

 cult to rob of their color. The most common decolor- 

 izer in use is probably alcohol — not absolute alcohol, 

 but alcohol containing more or less of water. Water 

 alone has this property, but in a much lower degree than 

 dilute alcohol. On the other hand, a much more ener- 

 getic decolorization than that possessed by either alone 

 can be obtained by alternate exposures to alcohol and 



1 111 the beginning of this chapter it was stated that the saturated alcoholic 

 solutions of the dyes do not serve as stains for bacteria. It must be remem- 

 bered that this holds only when absolute alcohol and perfectly dry coloring 

 matters have been used. If but a small proportion of water is present, the 

 bacteria may be stained with these solutions, though the results are, as a 

 rule, unsatisfactory. 



