The installation of a heating apparatus in middle-class houses is fairly common, and where one is not found many persons use gas or oil stoves in the passages in the winter, for it is now realised that it is not possible to heat rooms by means of open fires without creating cold draughts in them from the cold passages into which they open. And, moreover, the constant change of temperature encountered in passing from one warm room to another through cold passages is not only disagreeable, but is not found to be conducive to health. Let us turn to the cooling of water. Water expands about one-eleventh of its volume on becoming ice. This change of state, like that of change into steam, is accompanied by the evolution of tremendous mechanical force. If water freezes in pipes it bursts the pipes, and on a thaw taking place the pipes are found to leak. The appropriate remedy for this state of things is to protect the pipes from cold or to empty them when a frost is apprehended. In all properly built houses there is a tap by means of which the water supply can be cut off from the house, thus allowing the pipes to be emptied on a frosty night. The custom of leaving the taps dripping is effective, because the pipe is generally liable to freeze at some particular point where it is in immediate contact with the cold air, probably in the unclosed chink where the pipe passes through the wall; keeping the water moving in the pipe prevents any part of it getting cold enough to freeze, but the practice should not be resorted to, as it wastes water. RADIANT HEAT [Illustration: FIG. 4.--Section of a Convex lens.] It is pleasant on a dry, still day in winter, when the ground is covered with crisp snow or glistens with hard frost, to feel the warmth of the sun's rays, and it is becoming quite a fashion for people of leisure to spend the winter months at the pleasure resorts amid the snow-laden mountains of Switzerland. It is a matter of some interest to inquire how it happens that the sun's rays are warm when the thermometer tells us that the temperature of the air is below freezing-point. There is an old and pretty experiment in which a burning glass is made of ice; it is not a difficult thing to do. If the scale-pan of an ordinary balance be made hot and be pressed against a slice of ice (the concave side of the scale-pan towards the ice), first on one side of the slice and then on the other, the ice can be formed into a convex lens (Fig. 4). If now this lens be placed in the path of a sunbeam and the light be brought to a focus, that is, to a bright spot on a piece of paper, the paper will be heated and will take fire while the lens through which the heat passes remains ice. From this we may surmise that the heat of the sun does not affect the medium through which it passes.