MICROSCOPICAL EXAMINATION. 203 
give up their stain. The details of staining tubercle 
bacilli will be found under Tuberculosis. 
Another differential method of staining which is 
very commonly employed is that known as Gram’s 
method. In this method the objects to be stained 
are covered with the aniline gentian-violet solution. 
After remaining in this for a few minutes they are 
immersed in an iodine solution, composed of iodine, 1 
grain; potassium iodide, 2 grains; distilled water, 300 
c.c. In this they remain for from one to two minutes. 
They are then transferred to alcohol and thoroughly 
rinsed. If the cover-glass as a whole still shows a 
violet color, it is again treated with the iodine solu- 
tion, followed by alcohol, and this is continued until 
no trace of violet color is visible to the naked eye. 
They may then be washed in water and examined, or 
a contrasting 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 — particularly the pneumococcus — and 
also in differentiating between varieties of bacteria, for 
some do and others do not retain their stain when put 
in the iodine solution for a suitable time. 
Another method of demonstrating the capsule is the 
glacial acetic acid method, as described by Welch: 
1. Cover the preparation with glacial acetic acid for a 
few seconds. 2. Drain off and replace with aniline 
gentian-violet solution; this is to be repeatedly added 
until all the acid is replaced. 3. Wash in 2 per cent. 
solution of sodium chloride and mount in the same. 
Staining the Spores. We have already noted that 
during certain stages in the growth of a number of 
bacteria spores are formed which refuse to take up color 
