Smoke also causes decay of interior paintings and tends to restrict the furniture and furnishings to the more somber hues. In smoky atmospheres the interiors of buildings are usually more or less dark, hence special efforts must be made to keep the paintings free from dust and soot films. But constant mechanical or chemical cleaning or washing lessen the brilliancy of the color effects. A smooth, varnish-like or glazed coating on mural decorations is favorable for purposes of cleaning, but inimical to the best artistic effects. The matt enables the colored designs to show off to the best advantage, but matt is absorbent and attracts smoke and dirt products and resists successful cleaning. In the case of paint on plaster the corrosive products in smoke (particularly sulphuric acid) penetrate through the paint so that it tends to scale off; while in the case of fresco the porous surface of the carbonate of lime provides opportunity for the tenacious lodgment of dirt and for the destructive action of the acids. The carbonate is transformed into gypsum. It is so expanded that it loses its binding power, while the painting-ground is also often destroyed (30). It should be remarked incidentally that housewives in smoky cities are much inclined to keep doors and windows shut in order to exclude soot and dirt. While this may be aesthetically commendable, it is none the less hygienically objectionable. Moreover, the great amount of extra labor required to keep houses clean in smoky cities consumes the vital reserve of many women and robs them of the leisure moments which are theirs by right and which are essential for bodily rest and mental recreation and improvement. Not only is smoke inimical to the highest aesthetic utilization of artificial coloration--particularly exterior coloration--but it obliterates the most richly colored panorama of nature, namely, the iridescent clouds of the sky. The situation in cities is bad enough without the smoke, for in large cities the tall buildings prevent many people from ever getting a glimpse of the beautiful cloudscapes of the sunset or the splendor of the sunrise. But when you add the smoke palls to the buildings you often effectually close all visual approach to the richly illumined and picturesquely silhouetted clouds of the day and the resplendent stars of the night. That this frequent obscuration of the sky limits the aesthetic resources of any community has been shown by the writer in the article to which reference has already been made (9).