STAINING TUBERCLE BACILLI IN TISSUES. 195 



sections in contact -with their surface, after which they 

 are set aside in a place protected from dust, to remain 

 until the sections are firmly cemented to them by the 

 drying of the gelatin solution. The drying may be 

 hastened by keeping the slides in an oven below the 

 melting-point of the paraffin ; but it is best to set the 

 slides aside until the next day, when the sections will 

 be found to be perfectly cemented to them. The par- 

 affin is then removed from the sections by turpentine, 

 the tui'pentine by absolute alcohol, the absolute alcohol 

 by 50 per cent, alcohol, and this by water, after which 

 the slides are placed for five minutes in a 5 per cent, 

 aqueous solution of potassium bichromate. This ren- 

 ders the gelatin insoluble and prevents the sections 

 leaving the slides during their necessarily more or less 

 prolonged immersion in the fuchsin stain. The potas- 

 sium bichromate is washed out with water, and the slides 

 are then placed in a fuchsin stain, which is prepared as 

 follows : 



Dissolve the fuchsin in the alcohol and the carbolic 

 acid in the water. Mix the two solutions and let stand 

 for twelve hours, with occasional shaking or stirring ; 

 then filter. 



The length of time that the slide remains in this solu- 

 tion varies with circumstances. Tubercle bacilli stain 

 very quickly ; in tissues properly hardened in alcohol 

 five minutes are generally sufficient to stain them deeply. 



Prolonged immersion in the fuchsin does no harm 

 and insures certainty of results. After a section has 



