. Elements of chemistry ... , have alsoan important influence over their power of radiating caloric. 47. When other circumstances are equal, the rate atwhich bodies cool appears to be in an inverse ratio to thepolish, or brightness of their surfaces. Thus, the surfacesof bodies are found to radiate heat more rapidly when theyare rough than when smooth, and most rapidly when theirsurfaces are both rough and dark colo] ed. 48. Mr. Leslie covered one side of a cubical tin vesselwith lampblack; another side with writing paper; a thirdwith glass, and left the fourth uncovered. The vessel was then f
. Elements of chemistry ... , have alsoan important influence over their power of radiating caloric. 47. When other circumstances are equal, the rate atwhich bodies cool appears to be in an inverse ratio to thepolish, or brightness of their surfaces. Thus, the surfacesof bodies are found to radiate heat more rapidly when theyare rough than when smooth, and most rapidly when theirsurfaces are both rough and dark colo] ed. 48. Mr. Leslie covered one side of a cubical tin vesselwith lampblack; another side with writing paper; a thirdwith glass, and left the fourth uncovered. The vessel was then filled with hot water, and placedbefore a concave mirror, in the focus of which was placedan air thermometer, as re- presented by Fig. 10. Onturning the black side to-wards the reflector, thefluid in the thermometerindicated a rise of tempera-ture equal to 100 degrees ;the papered side beingturned towards the reflec-tor, the thermometer sunkto 98 degrees; the glassside indicated 90 degrees,and the metallic side only Fig. What is the difference between a mirror of glass, and one of metal, in theirpowers to reflect heat and light ? Why do polished andirons remain cold,when near the fire? Why is it difficult to boil water in a bright metallicvessel ? What effect does the nature, or eolor of * surface, have on its radia-ting power7 TRANSMISSION OF HEAT 33 12 degrees. The radiating power of these surfaces, there-fore, are respectively to each other, as the numbers 100, 98,90, and 12. Various practical uses may be made of this principle inthe common concerns of life. 49. A close stove, intended to warm a room by radiatingits heat to the objects surrounding it, should be dark col-ored, with a rough surface; while one intended to warmwith hot air passing through it, should have a bright, me-tallic surface. A dark, rough stove pipe, passing througha room, might render it comfortably warm; while a po-lished tin pipe, of the same length and dimensions, wouldhardly change its temperatu
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