PRACTICAL BACTERIOLOGY. 



thing wliich is still more injurious. The sulphuric acid 

 adds very much to the staining power of the methy- 

 lene blue, and hence the partially, or entirely, de- 

 colourised tubercle bacilli may become coloured violet 

 or light blue. Thus if there should be only a few 

 present, and if these offered only a comparatively 

 slight resistance to the decolourisation, they might 

 possibly be, by this method, completely overlooked, 

 as they would appear against the darker blue back- 

 ground only as light blue rods which might just as 

 well belong to any other slender species of bacillus. 



Similarly there are great objections to the method 

 of allowing the layer of sputum to dry on a slide, and 

 of fixing and staining it there. Much more heat must, 

 of course, be employed in order to fix a slide prepara- 

 tion than is necessary if a thin cover-glass be used ; 

 and as in addition the slide retains the heat much 

 longer, there is too much of a good thing. The bacilli 

 become overheated, and in consequence lose partially, 

 or entirely, their power of absorbing the stain, and 

 hence it is impossible to obtain really good prepara- 

 tions by this means. 



In the place of the 5 per cent, solution of sulphuric 

 acid, a 3 per cent, solution of hydrochloric acid in 

 absolute alcohol may be used. A 20 per cent, solu- 

 tion of nitric acid has a wonderfully quick effect ; some- 

 times a more dilute solution answers the purpose. 

 Nitric acid, however, must always be used with great 



