GENERAL BACTERIOLOGY 
87 
tilled water and the mixture is filtered through moist 
filter paper. 
To 108 c. c. of the mixture add 12 c. c. of the saturated 
alcoholic solution of gentian violet. 
3. Pour off the stain and cover it for thirty seconds 
with Gram’s iodine solution, which is prepared as fol¬ 
lows : 
Iodine .1 gram 
Potassium iodide .2 grams 
Distilled water..300 c.c. 
4. Pour off the iodine solution and decolorize with ab¬ 
solute alcohol until it ceases to discharge the blue color. 
5. Wash the preparation in water, and leave some water 
on the slide. 
6. Pour a few drops of carbol-fuchsin stain and im¬ 
mediately wash the preparation in water. 
7. Dry on a filter paper and put a drop of cedar oil. 
The carbol-fuchsin stain is prepared as follows: Dis¬ 
solve 1 gram of basic fuchsin in 10 c. c. of absolute alcohol, 
and mix the 10 c.c. of this alcoholic solution of fuchsin 
with 90 c. c. of 5 per cent aqueous solution of carbolic 
acid. 
When stained by this method, it will be found that 
some bacteria have retained the gentian violet stain, 
while others have lost it and have taken up the carbol- 
fuchsin stain. Those bacteria which, when stained by 
this method, have retained the gentian violet (i. e., are 
stained violet) are called Gram-positive, while those 
which have lost it and have taken up the carbol-fuchsin 
(i. e., are stained red) are called Gram-negative. The 
following table shows the most important bacteria as 
belonging to either one or the other class: 
