Page:Popular Science Monthly Volume 87.djvu/249

Rh beauty of the outline, causing considerable alteration of the original forms and making the building degenerate into a mere mass of dirty, shabby masonry.

It is rather interesting to note the vogue of such materials in buildings as glazed brick and terra cotta, in smoky cities. Stone, for which they are substitutes in many cases, is injured by smoke and the associated products of combustion in two ways:

1. The soiling of the surface, so that in the course of a few months all stone takes on the same gray, grimy color. This means either the loss of all artistic effect that may have been obtained by color contrast, etc., or the frequent cleaning of the building, which is an additional expense and at the same time aids in the destruction of the stone.

2. The actual destructive action on all stones by the acid products of the combustion of the coal. This is especially marked on any stone containing calcium or magnesium carbonates in larger or smaller quantities. Although there is some action on other constituents of the building stones, it is so slow in most cases as to be hardly noticeable.

A study was made in connection with the smoke investigation of the Mellon Institute as to the effect of smoke on outside painting. Both the effect on paint as a protective coating and as a decorative covering was considered. It was found that some paints last comparatively longer in Pittsburgh than in many other cities. This is in part due to the protective action of the soot in preventing the destructive action of the active rays of the sun. This would also indicate that such paints might afford a lodging place for fungi, which grow in the absence of direct sunlight.

It was found that smoke darkens paint coatings very rapidly and renders the use of light colors unsatisfactory on account of the short time they retain their true color. Analyses of the surface of the paints showed that the darkening was due to sulphur dioxide gas and to the accumulation of soot, carbon and similar organic matter contained in smoke.

The effect of smoke on metals must be taken into consideration in connection with the other building materials. When soot containing tar comes in contact with a metallic surface, it is made to adhere more or less firmly by means of its tar content. The occluded acids, principally sulphuric and sulphurous, are thus brought in intimate contact with the metal, giving a much better chance for corrosive action to take place and for it to become complete quicker than if the same amount of acid dissolved in rain water came in contact with the metal. The acid in the rain water drains off readily, while the soot protects the occluded acid to a great extent, so that it remains in contact with the metal until it has all been used up.