Those with rough surfaces absorb heat the best. What kind of objects absorb heat the least? Those with bright and smooth surfaces. What colored objects absorb heat the best? Objects of a dark or black color. Do objects that absorb heat well, radiate well? Yes; good absorbers are good radiators. Why is this? Because every substance must absorb heat before it can have heat to give off. Do objects that absorb heat well, reflect it well? No; objects that absorb heat well, cannot reflect it well. Why is this? Because a ray of heat cannot be reflected from an object, and be absorbed by that object, both at the same time. Why is water rapidly heated in a tea-kettle? Because the rough, dark iron of the tea-kettle absorbs the heat from the fire, and gives it off rapidly to the water contained in the kettle. The iron is also a good conductor of heat; therefore, the heat from the fire passes rapidly through it to the water. Why does black cloth, when spread on the snow, cause the snow to melt more rapidly? Because the cloth absorbs heat from the rays of the sun, and, therefore, melts the snow beneath it. Why does not white muslin, when spread on the snow, hasten its melting? Because the white muslin reflects the heat from the rays of the sun, so that but little of it enters the snow. Does the snow receive much heat from the sun? Snow is a good reflector, and throws back most of the heat from the sun. What causes the snow to melt? The heat it receives from touching the earth, causes the most of it to melt. Why does snow melt first on dark earth? Because dark earth is a good radiator, and gives off much heat to the snow. Why does snow remain longest on light-colored earth? Because the light-colored earth is a poor radiator, and gives off but little heat to the snow. Why does the air feel cold in winter time? Because it is so much cooler than our bodies that it absorbs heat from them rapidly, and thus makes them feel cold. Why does the air feel warm in summer time? Because it is then nearly as warm as our bodies, and, therefore, absorbs heat from them slowly. Why are greasy shoes warmer than those polished with blacking? Because the greasy shoes absorb heat from the sun, but the polished shoes throw off the heat of the sun by reflection. What may we remember about heat? And God said, Let there be light; and there was light. But along with the light, came its companion—heat; for when the sunbeam first came forth at His command, the twin sisters, light and heat, together touched the earth, causing it to bloom with life and beauty. And the earth brought forth grass, the herb yielded its seed, and the tree its fruit: And God saw that it was all good. [Illustration: An outdoor walk.] [Illustration: Decoration.] CHAPTER IV. The Air. [Illustration: Illustrated Capital W] Where do we find air?