Fig. 78 shows the arrangement of a very remarkable experiment, but little known, on the refreezing of ice. A block of ice is placed on the edge of two iron chairs, and is encircled by a piece of wire, to which is suspended the weight of say five pounds. The wire penetrates slowly, and in about an hour’s time has passed completely through the lump of ice, and the weight, with the piece of wire, falls to the ground. What happens then to the block of ice?—You imagine, doubtless, that it is cut in two. No such thing; it is intact, and in a single lump as it was previous to the experiment. In proportion as the wire was sunk through the mass, the slit has been closed again by refreezing. Ice or snow during the winter may serve for a number of experiments relating to heat. If we wish to demonstrate the influence of colours on radiation, we take two pieces of cloth of the same size,—one white, and the other black,—and place them both on the snow, if possible, when there is a gleam of sunlight. In a short time it will be found that the snow underneath the black cloth has melted to a much greater extent than that beneath the white cloth, because black absorbs heat more than white, which, on the contrary, has a tendency to reflect it. We perceive very plainly the difference in temperature by touching the two cloths. The white cloth feels cold in comparison with the black cloth. It is hardly necessary to point out experiments on the expansion of bodies. They can be performed in a number of different ways; by placing water in a narrow-necked bottle, and warming it over the fire, we can ascertain the expansion of liquids under the influence of heat. We may in this way construct a complete thermometer. We may now consider the _Sources of Heat_, or causes of its development, which are various, and in many cases apparent. The first great source is the Sun, and it has been calculated that the heat received by the earth in one year is sufficient to melt an envelope of ice surrounding it one hundred and five feet thick. Of course the heat at the surface of the sun is enormously greater than this, about one-half being absorbed in the atmosphere before it reaches us at all. In fact, it is impossible to give you an idea of the enormous heat given out by the sun to the earth (which is a _very_ small fraction indeed of the whole), stars, and planets, all of which give out heat. We know that heat is stored in the earth, and that it is in a very active condition we can perceive from the hot springs, lava, and flame which are continually erupting from the earth in various places. These sources of heat are beyond our control.