144 SA GTERIOLOG V. 



Yery dilute acetic acid robs tissues and bacteria of 

 their staining with remarkable activity ; still more ener- 

 getic 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 that are commonly employed are : 



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

 solution. 



Nitric acid in from 20 per cent, to 30 per cent, watery 

 solution. 



Hydrochloric acid in 3 per cent, solution in alcohol. 



STAINING OF BACTERIA IN TISSUES. 



In staining tissues for the purpose of demonstrating 

 the bacteria which they may contain, a number of points 

 must be borne in mind : the conditions which favor the 

 diffusion of the staining fluids into the bacteria are now 

 not so favorable to rapid staining as they were when 

 the bacteria alone were present upon cover-slips ; the 

 staining of tissues, therefore, requires a longer exposure 

 to the dyes than does that of cover-slips. In tissues, too, 

 there are other substances beside the bacteria which 

 become stained, and these, unless robbed in whole or in 

 part of their color, may so mask the stained bacteria as 

 to render them difficult, if not impossible of detection. 

 Tissues must, therefore, always be subjected to some 

 degree of decolorization, and this must be practised 

 without depriving the bacteria of their color. 



The details of the methods of decolorization will be 

 described in the section on the technique of staining. 



Another point to be remembered in staining tissues 

 is that they can never be heated and retain their struc- 



