230 TEXT-BOOK OF BACTERIOLOGY. 



It is needless to say that we proceed in the same way in making- 

 any other cover-glass preparation, of faeces, pus, or a pure culture 

 preparation. 



Cover-glasses thus prepared should be held only with forceps. 

 Cover the prepared slide with carbol-fuchsin and hold the cover- 

 g-lasses over a flame until evaporation takes place. Withdraw the 

 cover-glass in an instant, repeat the procedure several times, re- 

 placing if necessary the evaporated staining fluid. Now pour off 

 the staining fluid, wash the preparation with distilled water until 

 cleansed. 



Now follows the most important step of the process, namely, 

 decoloration. The coloring matter is retained with unusual tenac- 

 ity by the bacilli, but can be removed from the surrounding tis- 

 sue by means of a 15$ to 20$ solution of nitric acid. The cover- 

 glasses are moved to and fro in the acid solution a number of 

 seconds until the deep red color has changed to a greenish-blue. 



The preparation is next placed in a vessel of diluted (70$) alco- 

 hol for the purpose of removing the fuchsin which has been set free 

 by the acid. It leaves thfe preparation in thick red clouds, the 

 preparation becoming paler and paler until finally it remains as a 

 light film of a rosy hue. After removing from the alcohol the 

 cover-glass is washed with distilled water and then counter- stained 

 with the usual dilated methyl-blue solution. Those parts of the 

 preparation which under the- influence of the acid have been decol- 

 ored now take up the blue; the tubercle bacilli, on the contrary, re- 

 tain their red color, and consequently are seen with special dis- 

 tinctness. The various other bacteria in the same preparation are 

 colored blue, which distinguishes them at once from the tubercle 

 bacilli. 



The expectoration from consumptives is usually very rich in 

 foreign micro-organisms, especially streptococci and bacilli of de- 

 composition, and therefore it is necessary in each case to notice this 

 contrast carefully. 



The excess of methyl-blue is in a short time removed with water, 

 and the blue-looking preparation is either at once examined or else 

 dried and then mounted in Canada balsam. 



The first procedure is best, because by it we are able to complete 

 our examination more rapidly and overcome difficulties such as in- 

 sufficient decoloration, and because, furthermore, it retains the 

 shape of the bacilli best, which frequently shrink in the balsam or 

 subsequently lose their color. 



Where we do not wish to make permanent preparations, the 

 entire method may be shaped after the following concisely-described 



