ii6 PRACTICAL BACTERIOLOGY. 



By that time everything in the sputum which can 

 be affected by the solution is stained : tubercle bacilli, 

 other bacteria, pus cells, and all the other elements of 

 the animal organism. As we only wish the tubercle 

 bacilli to retain the red colour, it is necessary to de- 

 colourise all the rest as much as possible; alcohol 

 alone is not sufficiently powerful in this case, for the 

 - stain has been eagerly absorbed, not only by the many 

 other bacteria present in the sputum, but also by the 

 pus corpuscles and nuclei, and is, moreover, very firmly 

 retained by them. Of course, if the cover-glass were 

 left for several days, or even weeks, in 60% alcohol 

 solution, all these bodies would become decolourised ; 

 but this result may be much more quickly attained 

 by the use of 5% sulphuric acid in the place of the 

 alcohol. The cover-glass is left in the acid until the 

 preparation appears to be completely decolourised, 

 that is to say, for a few seconds (the time varying 

 according to the thickness of the layer) ; it is then 

 immediately immersed in alcohol, and rinsed in it 

 until no more stain comes away. If the preparation 

 be now examined in water with the highest available 

 power of the microscope, it is seen that the fine slender 

 tubercle bacilli are stained a deep red, and stand out 

 very clearly from the nearly colourless background. 



Nevertheless red stained objects, which from their 

 form are evidently not tubercle bacilli, are always pre- 

 sent as well. Sometimes they are substances which 



