=THEORIES OF STAINING.= The exact nature of the process of staining has not yet been determined. Various theories have been advanced, explaining the affinity of the tissues for certain dyes, on the ground of a chemical, mechanical or chemicophysical action. The chemical theory assumes the formation of an insoluble compound through the chemical combination of tissue and stain; the physical theory is based upon the assumption that the process is purely physical or mechanical, while the chemicophysical theory holds that it is neither purely physical nor purely chemical. The process is not controlled by the molecular weights alone of the substances concerned, but does depend upon the conditions controlling the formation of solutions in general. Therefore, the theory most widely accepted at the present time is the solution-theory, which assumes that the staining-process is a solution of the dye in the tissue, and that the stained tissue-element is a fixed solution of the stain in its substance. This solution of the dye in the tissues may be a direct action between the two (direct or substantive stains); or it may be brought about only by the interaction of a third substance (indirect or adjective stains). The third substance is called a _mordant_, and the combination of the dye with the mordant is known as a _lake_. The mordant may be added to the stain or to the tissue, either before or at the time of staining. Many of the fixing-fluids are mordants, particularly those containing chromic acid or its salts. Alum, iron, and many of the metals are the most commonly used mordants. In a general way acid mordants are used for basic colors, and basic mordants with acid colors. The =stains= most commonly used in pathologic work are:— 1. =Natural Dyes=:—Hæmatoxylin and Carmine. 2. =Aniline Dyes=:—_a, Acid._—Eosin, erythrosin, acid fuchsin, orange G, picric acid, sudan III, and scarlet R (Fett-ponceau). _b, Basic._—Methylene blue, methylene violet, thionin, toluidin blue, kresyl-echt-violett, methyl violet, gentian violet, crystal violet, basic fuchsin, dahlia, aniline blue, methyl green, iodine green, safranin, Bismarck brown, and vesuvin.