Page:International Library of Technology, Volume 93.djvu/38

 twice the length and breadth and, consequently, has four times the area of the plate a; and the centers of both plates lie on the straight line oc passing through the source of heat. The heat rays radiate in all directions from the flame, but always in straight lines; and, since straight lines passing through the center of the flame and the edge of the plate a also touch the edge of plate b, it follows that the same number of heat rays fall on the plate b as fall on the plate a. If the plate a were placed 2 feet from the flame, that is, in the position of the plate b, it would receive only one-fourth the number of heat rays that previously fell on b, since it has but one-fourth the area of b. Now, the amount of heat transmitted is directly proportional to the number of heat rays. Consequently, by moving the plate twice as far away from the flame, it receives only one-fourth the amount of radiant heat; if it had been moved 3 feet away, it would have received only one-ninth the heat; and at a distance of 4 feet, it would receive but one-sixteenth the amount of heat that it received at a distance of 1 foot. Hence, the intensity of radiant heat, or the number of heat rays falling on a given area, varies inversely as the square of the distance from the source of heat.

24. Reflection and Absorption of Heat. — When radiant heat falls on the surface of a body, a part of the heat is reflected and the remainder is absorbed. The condition of the surface affects largely the proportion of heat reflected or absorbed. Thus, a highly polished surface will reflect most of the heat that falls on it and will absorb very little, while a dull rough surface will absorb much, and reflect little, of the heat. A surface that reflects much heat will not radiate much heat. A brightly polished teakettle filled with hot water will remain hot for a longer time than a dull and tarnished kettle. In the same way, a surface that absorbs heat readily will also radiate heat readily. For example, a stove coated with lampblack will give out heat much more rapidly than if it had a polished nickel surface, because lampblack radiates heat more readily than any other known substance. It likewise readily absorbs heat.