4 MICRO-ORGANISMS AND FERMENTATION. 



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 decolourised material. 

 EHRLICH carries out the staining in the following manner : 

 Finely-powdered gentian-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. 



Another method of detecting these bacteria, which are of 

 such frequent occurrence, was recently given by GABBET : 

 Strong carbolic fuchsine is poured over the cover-glass and the 

 latter heated by holding it for half a minute over a flame ; then 

 the fuchsine solution is poured off and replaced by a few drops 

 of a dark-coloured solution of methylene blue in 25 per cent, 

 sulphuric acid, which solution will in the course of some hours 

 discolour and again colour the preparation. The latter is 

 washed, and then examined in water. 



As is well known, photographic illustrations of bacteria 

 have recently come into general use, having been first intro- 

 duced by KOCH. 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 seldom mixed 

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

 led to the discovery of specific characters (Bacterium aceti and 

 B. Pasteurianum, see Chapter III.). 



On the other hand it is sometimes necessary in the 

 examination of the organisms of fermentation, and especially 

 of bacteria, to adopt another method of preparation. The 



