ORDINARY STAINING-SOLUTIONS 167 



When so treated, certain of the bacteria will retain their 

 color, even when exposed to very strong decolorizers. The 

 tubercle bacillus is distinguished from the great majority 

 of other bacteria by the tenacity with which it retains the 

 color when treated in this way; it is an organism difficult 

 to stain, but when once stained is equally difficult to rob of 

 its color. 



Decoloeizing-solutions. — ^As regards the employment 

 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 made to take up the staining-material 

 only with difficulty are also very difficult to rob of their 

 color. The most common decolorizer 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 energetic 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 diffuse 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 energetic are 

 solutions of the mineral acids, and particularly, as has been 

 said, when this action is accompanied by the decolorizing- 

 properties of alcohol. 



The acid solutions commonly employed are: 



Acetic acid in from 0.1 to 5 per cent, watery solution. 



