358 THE POPULAR SCIENCE MONTHLY.
per cent, as compared with the sky light of a clear day. When it was in shadow the intensity fell to only ten per cent.
The brownstone, so familiar in building construction, is justly looked upon as a gloomy material for the purpose. Connecticut stone is the darkest, and, especially when oiled, is a somber thing with which to enliven architecture. But even when oiled it does not absorb quite all the light which strikes it. The searching light of the sun will find many little particles among the grains of the exterior that give surface reflection, and this white light mingles with the dark interior hue of the stone to brighten it considerably. The Connecticut stone under ordinary conditions gives some sixty to seventy per cent of the intensity of sky light, when the sun shines brightly upon it. Belleville stone is much lighter, and has been shown to exceed the sky light by twenty or thirty per cent when illuminated by the sun.
Among the bricks there is a large opportunity for choice in their capacity for reflecting light. The Philadelphia pressed brick is popular for its richness of color, but deserves no prominence for its reflecting power. In that respect the coarser Hudson River brick is an improvement. There is, of course, considerable vari- ety, but it may in general be said that walls of Philadelphia brick, and those painted red to imitate that shade, will reflect sunlight to an extent varying between fifty and eighty per cent of ordinary sky light. When illumined by only indirect light from the sky or other sources they have an intensity of fifteen to twenty per cent of the same sky light. With rough brick walls there is al- ways a considerable show of light-colored mortar, and this, with the lighter surface of the bricks themselves, causes a greater re- flecting power. It is easy to find surfaces of this character with a reflecting power when illuminated of ninety to one hundred per cent.
Buff-colored bricks make admirable reflecting surfaces. With the sun upon such a surface it will often reflect one hundred and sixty per cent or more of sky-light intensity. Many opportunities will offer in modern construction for the observer to bring such an illuminated surface against a sky background, and see how much brighter it will ordinarily be. Its practical value as a re- flector is greatly lowered in his estimation, however, when he finds that, if the sun is not shining upon it, twenty-five per cent is a liberal estimate of its powers of reflection.
The climax of reflecting power in brickwork is reached in the cream and white enamel brick that are in the market. They are chiefly used for interior work, and their f)olished surfaces place them in a different class of reflectors from the ordinary building materials.
Slate roofs belong to the same class in a certain degree. The