Two or three drops of alcoholic solution of these dyes to, say, 4 c.c. water, usually makes a sufficiently strong staining fluid for cover-slip film preparations. Carbolic methylene-blue (C.M.B.) and carbol fuchsin (C.F.) are prepared by covering the cover-slip with 5 per cent. solution of carbolic acid and adding a few drops of the saturated alcoholic solution of methylene-blue or fuchsin respectively to it. For aniline gentian violet (A.G.V.) the stain is added to a saturated solution of aniline oil in water. (d) Thionine blue. (e) Bismarck brown. (f) Neutral red. (B) Acid: (a) Eosin, aqueous yellowish. (b) Safranine. These dyes are kept in 1 per cent. aqueous solution to which is added 5 per cent. of alcohol, as a preservative. They are generally used in this form. A few nuclear stains (carmine, hæmatoxylin) are occasionally used more especially in "section" work. _Decolourisation._--After overstaining, films may be decolourised by washing for a longer or shorter time in one of the following reagents arranged in ascending order of power 1. Water. 2. Chloroform. 3. Acetic acid, 1 per cent. 4. Alcohol. 5. Alcohol absolute, } equal parts. Acetic acid, 1 per cent., } {Hydrochloric, 1 per cent. aqueous solution. {Hydrochloric, 1 per cent. Alcoholic { (90 per cent.) solution. 6. Mineral acids: {Sulphuric, 25 per cent. aqueous solution. {Nitric, 33 per cent. aqueous solution. _Counterstaining._--Use colours which will contrast with the first stain; e. g., Vesuvin, } Neutral red, }for films stained by methylene-blue or Eosin, }Gram's method. Fuchsin, } Methylene-blue, }for films stained by fuchsin. Gentian violet, } 8. _Mounting._-- 1. Wash the film carefully in running water. 2. Blot off the superfluous water with the filter paper, or dry more completely between two folds of blotting paper. 3. Complete the drying in the air, or by holding the cover-slip in the fingers at a safe distance above the flame of the Bunsen burner. 4. Place a drop of xylol balsam on the centre of a clean 3 by 1 glass slide and invert the cover-slip over the balsam, and lower it carefully to avoid the inclusion of air bubbles. NOTE.--Xylol is used in preference to chloroform to dissolve Canada balsam, as it does not decolourise the specimen. ~Impression films~ (_Klatschpraeparat_) are prepared from isolated colonies of bacteria in order that their characteristic formation may be examined by higher powers than can be brought to bear on the living cultivation. They are prepared from plate cultivations (_vide_ page 230) in the following manner. 1. Remove a clean cover-slip from the alcohol pot with sterile forceps and burn off the spirit.