98 BACTERIOLOGY. 



In Ehrlich's method delicate sections are liable to be injured by 

 immersion in the nitric acid, and therefore Watson- Oheyne suggested 

 the use of formic acid. ' 



The Ziehl-Neelsen method, in which sulphuric acid is used instead 

 of nitric acid, is much to be preferred to Ehrlich's method. 



Ziehl-Neelsen Method. The solution is warmed, and sections 

 left in it for ten minutes. The red colour, which disappears when 

 the section is placed in weak sulphuric acid (25 per cent.), may 

 partly return when the section is placed in water. In this case the 

 section must be again immersed in acid and passed backwards and 

 forwards from acid to water until the red colour has completely, or 

 almost completely, disappeared. It must be thoroughly washed in 

 water to remove all traces of the acid, and then placed in a watery 

 solution of methylene blue for two or three minutes, washed again 

 in water, immersed in alcohol, clarified in clove-oil, and mounted in 

 the usual way. Sections are brilliantly stained, and the results are 

 very permanent. 



Many special methods of staining have been introduced, and will 

 be given in subsequent chapters with the description of the bacteria 

 to which they apply. The methods already described are those 

 which are more or less in constant use in studying bacteria and in 

 conducting original researches. 



