4 MICRO-ORGANISMS AND FERMENTATION. 



bacilli are surrounded by a cell- wall which is only permeable 

 by alkaline liquids. Therefore, when the bacilli, cell-nuclei, 

 plasma, etc., are stained by the alkaline solution, and the 

 first -named are consequently practically indistinguishable in 

 the mixture, treatment with an acid removes the stain from 

 all the other parts of the section and from all foreign 

 organisms ; but, as the presumed envelope of the tubercle- 

 bacilli cannot be penetrated by the acid, these bacilli will 

 remain as the only stained bodies in the otherwise entirely 

 decolourised material. Ehrlich carries out the staining in the 

 following manner : Finely-powdered gentiana-violet is 

 dissolved in a saturated aqueous solution of aniline ; 10 to 20 

 drops of this solution are filtered into a watch-glass, in 

 which the section to be examined is allowed to remain for 

 about 24 hours. It is then rinsed with distilled water and 

 again placed in the watch-glass with a solution of 3 parts 

 of nitric acid in 100 parts of alcohol. After three to five 

 minutes the section is decolourised ; it is then transferred to 

 pure alcohol, and finally examined in clove oil. 



As is known, photographic illustrations of bacteria have 

 recently come into general use, having been first introduced 

 by R. Koch. In order to obtain these, staining and 

 decoloration are quite necessary, partly in order to render 

 the contours of the bacteria sharper, and partly in order to 

 remove all bodies detrimental to the picture. 



Staining and decoloration are not generally required in 

 investigations connected with the physiology of fermentation, 

 where the organisms are almost always free, and only seldom 

 mixed with disturbing elements, and only in a few cases has 

 staining led to the discovery of specific characters (Bacterium 

 aceti and B. Pasteurianuin, see Chapter III.). 



On the other hand, however, it is sometimes necessary 

 in the examination of the organisms of fermentation, and 

 especially of bacteria, to adopt another method of preparation. 

 The particles of organic and inorganic matter which separate 

 from the solutions frequently have a deceptive similarity to 



