STAINING THE TUBERCLE BACILLUS. 161 



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 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 commonly employed are : 



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



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

 tion. 



Sulphuric acid in from 5 to 10 per cent, solution in 

 water. 



Hydrochloric acid in from 1 to 3 per cent, solution in 

 alcohol. 



NOTE. For details as to the technique of hardening 

 and cutting sections and staining bacteria in tissues, the 

 student is referred to Mallory and Wright's Pathological 

 Technique. 



METHOD OF STAINING THE TUBERCLE BACILLUS. 

 Select from the sputum of a tuberculous subject one 

 of the small, white, cheesy masses which it is seen 

 to contain. Spread this upon a cover-slip, dry and 

 fix it in the usual way. The slip is now to be taken 

 by its edge with forceps and the film covered with a 

 few drops of either the solution of Koch-Ehrlich or 

 that of Ziehl. It is then held over a gas-flame, at first 



