In a general way it may be said that basic stains are nuclear stains, and acid stains are protoplasmic. Neutral stains are usually diffuse stains; when formed by the combination of acid and basic dyes they usually act as selective stains for some especial tissue-element or cell-constituent. =Metachromasia=, in its narrowest sense, is the term applied to that staining-phenomenon, in which a single-chemical entity gives different colors to different tissue-elements. In this sense iodine, in its action upon glycogen and amyloid, is a true metachromatic substance. The majority of the so-called metachromatic stains, however, do not possess true metachromasia, since their metachromatic powers are dependent upon a mixture of two or more dye-stuffs in the one compound. The most important stains of this class are gentian violet, methyl violet, crystal violet, dahlia, thionin, toluidin-blue, polychrome methylene blue, methylene azure, and kresyl-echt-violett. The chief _chromotropic_ substances are amyloid, mucin, mast-cell granules and cartilage. Metachromatic reactions are at their best usually when examined in water; they are affected by alcohol and usually destroyed by carbol-xylol. Sections stained by metachromatic stains should be quickly dehydrated by absolute alcohol and blotting, and cleared in xylol, when mounted in balsam. I. NUCLEAR STAINS. 1. =Haematoxylin= (C_{16}H_{14}O_{6}) is an ether extract of the wood of _Hæmatoxylon campechianum_, a tree found in the West Indies and Central America. In itself not a dye, it becomes one of the most valuable when oxidized to hæmatein (C_{16}H_{12}O_{6}), and combined with alum, iron or other mordants to form a lake. It then stains nuclei a deep violet-blue or black color that is practically permanent. Mucin, lime-salts, bacteria, and colonies of actinomyces are also stained varying shades of blue. If the staining process is prolonged the entire tissue, as well as celloidin, becomes more or less heavily stained blue. A pure nuclear stain is obtained by interrupting the stain at the right time (examine in water); or if the sections are over-stained they may be differentiated in acid alcohol (1 per cent hydrochloric acid in 70 per cent alcohol). Hæmatoxylin stains well after all fixing-solutions except osmic acid; some of its staining-formulæ stain slowly after fixation in Zenker’s fluid. On the whole, it is by far the best general nuclear stain for laboratory and diagnostic work. It is employed in numerous staining formulæ, the most useful of which are given here. These formulæ differ chiefly in the time of staining, “ripening” of the stain (oxidation of hæmatein), intensity of stain, necessity of differentiation, etc. a. =Böhmer’s Alum-haematoxylin.=