178 BACTERIOLOGY, 



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, 

 alcoholic solutions of the stain ing-dyes cannot be satis- 

 factorily employed. The stain ing-dyes should always 

 be watery. 1 



DECOLORIZING-SOLUTIONS. As regards the employ- 

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

 mind that objects which are easily stained are also easily 

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

 staining-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 less 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 

 water. More energetic in their decolorizing action than 

 either water or alcohol are solutions of the acids. They 

 appear, particularly when they are alcoholic solutions, 

 to diifuse rapidly into tissues and bacteria and very 

 quickly extract the staining-materials which have been 

 deposited there. For this reason these solutions should 

 be employed with much care. 



Very dilute acetic acid robs tissues and bacteria of 

 their stain with remarkable activity ; still more ener- 

 getic are solutions of the mineral acids, and particularly, 



1 In 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 remembered that this holds only when absolute alcohol and 

 perfectly dry coloring-matters have been used. If but a small propor- 

 tion of water is present, the bacteria may be stained with these solu- 

 tions, though the results are, as a rule, unsatisfactory. 



