Exterior painting in the form of mosaic or polychromic ornamentation cannot be successfully employed to any extent in smoky cities (32). Colored effects on stones, metals, ordinary or glazed bricks or faience will not for long retain their purity in a murky atmosphere. A grimy, blotchy, streaky, inartistic, abject appearance of the exterior chromatic decoration of buildings in smoky cities is a matter of common observation. Even glazed white bricks when used inside the building may become dirty and discolored. Observation shows that placards in red lead become gradually decolored in cities, even when protected against the sun and rain, but not in the country air (35). The pernicious influence of the smoke on color effects is probably due to the fact that the colors become mechanically covered over by a coat of tarry soot and dirt. This tarry mass may effectually obliterate all color effects. It tends to adhere so firmly that it can only be removed, if at all, with great difficulty and with the aid of special solvents, and in the process of removal the colored paints often tend to scale off with the dirt. The defiling effect of the actual dirt is the most objectionable influence of smoke on stained glass. But smoke may also cause the disintegration of paint films, owing to the corrosive agents which smoke often contains. It is thus evident that smoke largely restricts exterior coloration to a play of light and shade. In fact, it tends to restrict paint effects to the darker shades.