stain

[M.E. sieinen:\ i. To discolor. 2. To color, to dye. 3. discoloration. 4. dye used in histological and bacteriological technique, acid s., an aniline salt in which the acid is the staining part, such as the picrates, eosin, acid fuchsin, Congo, etc.; the qualification "acid" has no reference to the litmus reaction and is not to be taken in its chemical sense; acid stains affect chiefly the protoplasm, basic s., an aniline salt (usually neutral in its litmus reaction) in which the basic element is the staining part, such as basic fuchsin, methylene blue, Bismarck brown,