152 BACTERIOLOGY. 
ground water has filtered out the germs that were in the water that 
trickled through it. 
What methods are in use to free drinking water from all bacterial 
dangers? 
Filters of sand, carbon and unglazed porcelain have been used, but 
they are not absolutely certain in their action. 
To be absolutely sure that all bacterial life in water has been dis- 
troyed, it must be thoroughly boiled. 
STAINING. 
What dyes are used in staining bacteria ? 
Many of the aniline dyes (which are produced from coal tar) 
have been used, but the ones most commonly depended upon are 
Suchsin, gentian violet, methyl violet, methylin blue, and Bismarck 
brown, Sometimes an acid fuchsin is used. 
How are these dyes prepared for ordinary work ? 
Most of them are more soluble in alcohol than in water, Bismarck 
brown being an exception. Filtered saturated alcoholic solutions of 
the dyes are made and kept as stock solutions. These stock solutions 
are never used directly as staining agents, as they give too deep colors. 
To use them, ordinary clean water or distilled water is colored with the 
stock solution to be used. This watery solution should be colored so 
deeply that it is just transparent. 
If there seems to be an excess of color, what methods are adopted 
to obviate this? 
The stained specimen may be washed either with water slightly 
acidulated with a few drops of acetic acid or with strong alcohol, or 
with Gram’s solution. 
What is the formula of Gram’s solution, and how is it used ? 
It is a solution of iodine 1 part, potassium iodide 2 parts, distilled 
water 300 parts. The dye is prepared by coloring a saturated watery 
solution of aniline oil with the stock solution of gentian violet, added 
just to the point of saturation. After the specimen is colored with 
this it is placed in the Gram’s solution and then washed with water. 
The structural elements of the specimen are thus decolorized and the 
bacteria stand out clear and plain, 
What is a mordant ? 
It is a substance that when added to or used with a dye causes the 
staining to become fixed in the material so stained. 
