Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

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, the effect must be attributed to the radiation of heat from the thermometer to the ice. If a hot ball be placed in the focus of one mirror and a piece of phosphorus in that of the other, the phosphorus will be set on fire, though the mirrors may be twenty or more feet apart.

Fig. 218.

The reflection of heat may be exhibited very prettily with the experiment represented in Fig. 218. A sheet of bright gilt paper is rolled up in the shape of a funnel, with the metallic side inward. Holding the larger end toward a fire, the rays of heat coming from the fire into the funnel are reflected toward a central line, and so pass out of the smaller end of the funnel concentrated. If, now, a bit of phosphorus or a lucifer match be held a little distance from this end of the funnel it will be set on fire.

315. Formation of Dew.—It is by the radiation of heat that dew is formed. The earth is constantly radiating heat into space as well as the sun. In the daytime it receives a great deal more than it radiates. But at night this is reversed, and the earth is cooled. The cooled earth condenses the moisture in the air which is in contact with it, and so the moisture is deposited. If the weather be very cold this is frozen, and then we have frost instead of dew. You observe that the dew does not fall, though this is the ordinary expression. Its formation is analogous to the deposit of moisture which we so often witness in a hot day in summer on the outside of a tumbler containing cold water. As the cold tumbler condenses the moisture in the air, so does the earth at night, it being cooled by radiation, condense the moisture which has accumulated in the air by evaporation during the heat of the day.

There are some circumstances which have an influence upon the deposition of dew and frost. Less is deposited under a tree than outside of it, because all the heat which radiates vertically upward from under the tree is radiated back again by the tree. Hence the efficacy of a covering over plants as a defense against frost. Clouds operate in the same way, and for this reason no dew or frost is deposited in a cloudy night. Neither is any deposited in a very windy night, because the moving air promotes evaporation, and thus prevents the accumulation of moisture.

Dew is deposited in different amounts on different substances. This is owing to a difference in radiation. Grass and leaves radiate heat better than earth, and earth better than stone; and therefore while stones and gravel-walks may be dry or nearly so, the loose earth may be moist and the grass and leaves thoroughly wet. So you see that not even the dew, plentiful as it is, is wasted by the Creator, but is deposited just where it is wanted to refresh the parched earth and its vegetation.

316. Gideon's Fleece.—If you lay a fleece of wool upon the ground, it is so poor a radiator of heat that no dew will be deposited upon it, although there may be an abundance of it on the grass and leaves in its neighborhood. But this was reversed in the case of Gideon's fleece. The laws of nature were set aside, and the fleece was wet with dew while all around was dry.