STAINING THE TUBERCLE BACILLUS. 165 



After remaining in this for twenty to thirty minutes 

 they are immersed in a solution composed of 



Iodine 1 gramme. 



Potassium iodide 2 grammes. 



Distilled water 300 c.c. 



In this they remain for about five minutes ; they are 

 then transferred to alcohol and thoroughly rinsed. If 

 still of a violet color, they are again treated with the 

 iodine solution, followed by alcohol, and this is con- 

 tinued until no trace of violet is visible to the naked 

 eye. They may then be examined, or a contrast-color 

 of carmine or Bismarck-brown may be given them. 



This method is particularly useful in demonstrating 

 the capsule which is seen to surround some bacteria, 

 especially micrococcus lanceolatus of pneumonia. 



GLACIAL ACETIC ACID METHOD. Another method 

 that may be employed for demonstrating the presence 

 of the capsule surrounding certain organisms is to pre- 

 pare the cover-slips in the ordinary way, then cover the 

 layer of bacteria upon them with glacial acetic acid, 

 which is instantly poured off (not washed off in 'water), 

 and the aniline- water gentian-violet solution dropped 

 upon them ; this is allowed to remain three or four 

 minutes, is poured off, and a few drops more are added, 

 and lastly the slip is washed in a solution of sodium 

 chloride. Usually this is of the strength of physio- 

 logical salt-solution, viz., 0.6 to 0.7 per cent. ; but at 

 times it must be stronger, occasionally as concen- 

 trated as 1.5 to 2 per cent. The reason for this 

 is that if the slips be washed in water, or in salt- 

 solution that is too weak, the mucin capsule that 

 has been coagulated by the acetic acid is redissolved 



