35 8 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 polished 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 



