170 BAGTERIOLOOY. 



gentian-violet made after the formula of Kocli-Ehrlich. 

 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 



