MAKING COVER-GLASS PREPARATIONS 25 



36. Aqueous Solutions. — Aqueous solutions of methyl-vio- 

 let, gentian-violet, fuchsin, and the other aniline dyes are pre- 

 pared by adding i c.c. of the saturated alcoholic solution of the 

 desired dye to 20 c.c. of distilled water. This will impart a 

 decided color to the liquid so that a pipette full will be barely 

 transparent. 



The true aqueous solutions are made by dissolving the dyes 

 in water, but these are weak and not so effective as those pre- 

 pared from the alcoholic solutions. These solutions deteriorate 

 in a short time. The carbol fuchsin and alkaline methylene- 

 blue will keep a little longer, but they require to be filtered 

 occasionally. 



37. Making Aniline Water. — Aniline water is a saturated 

 aqueous solution of aniline oil. It is prepared by adding i c.c. 

 of aniline oil to 20 c.c. of distilled water and shaking frequently 

 for from 15 to 30 minutes. It is convenient to use a stoppered 

 vial or large test tube for mixing it. Filter through a moistened 

 filter paper. The filtrate should be perfectly clear. If it is 

 cloudy, it should be refiltered before using. This is used in 

 preparing the aniline water dyes, such as methyl-violet, gentian- 

 violet, etc. 



38. Gram's Method of Staining Bacteria. — Prepare the 

 cover-glass preparations as already described. Stain them in 

 gentian-violet aniline \Vater or in a saturated alcoholic solution 

 of gentian- violet in 5^ carbolic acid in the proportion of i to 

 20 for from 5 to 7 minutes. Rinse in water and transfer them 

 to a watch glass containing Gram's solution until the color 

 becomes quite black. This requires from i to 2 minutes ; 

 then place the preparations in a watch glass containing alcohol 

 and allow them to remain there until the color has almost 

 entirely disappeared, or has become a pale gray. Rinse in 

 water and examine at once, or allow them to dry and mount in 

 balsam. (Sections of tissues must be dehydrated and cleared 

 before mounting.) 



