It is otherwise with heat that comes from a common fire. It does not seem to be so thoroughly united with the light, and therefore readily parts company with it, as we may say. While the heat and light of the sun go together through all transparent bodies the heat of a fire will not go with its light through all of them. So while the heat of the sun does not warm the glass through which it passes the heat of a fire will warm it, and therefore glass is an effectual screen against it. In some operations in the arts a mask of glass is sometimes worn to ward off the heat. The connection of light and heat will be farther noticed when I come to treat of light. 313. =Relation between Radiation and Absorption.=--All surfaces that radiate will absorb also equally well the heat that is radiated upon them. All rough and dark surfaces both absorb and radiate freely; but all light-colored and polished surfaces do both slowly. For this reason the black, rough tea-kettle is well fitted to heat water in; but it is not fitted to retain the heat in the water. On the other hand, the bright, polished tea-pot absorbs heat poorly, but retains it well. [Illustration: Fig. 217.] 314. =Reflection of Heat.=--Radiated heat is reflected; and here, as in the case of motion, § 206, and of sound, § 260, the angles of incidence and reflection are equal. Some interesting experiments in relation to the reflection of heat can be tried with concave metallic mirrors. Thus, if we take two such mirrors, as represented in Fig. 217, and place in the focus of one a thermometer, and in the focus of the other a small flask of hot water, or a heated iron ball, the mercury in the thermometer will rise, although the mirrors may be many feet apart. Observe how the effect is produced. Rays of heat go from the flask directly toward the thermometer, as represented by the lines in the figure; but that the effect does not come from these can be proved by removing the mirrors, leaving the flask and thermometer just as they are. When the experiment is tried in this way no effect is produced on the thermometer, because it is too far from the source of heat, the flask, to receive any perceptible influence in this way. The effect comes from the rays of heat which go to the mirror near the flask, and are reflected to the other mirror, and then are reflected upon the thermometer, all of which is represented by the dotted lines. There is another way, besides that already mentioned, of showing that it is not the _direct_ rays that produce the effect. After arranging the apparatus put a screen between the thermometer and the mirror near it, and the effect will be prevented because the reflection is cut off. If a piece of ice be substituted for the flask of hot water the thermometer will fall--an effect opposite to that produced in the previous experiment. This would seem to show that cold is radiated, but as there is no such thing really as cold, § 270,