CHAPTER XII.

ON THE CONCENTRATION OF RAYS OF LIGHT AND HEAT AND ON THE LIMITS OF ITS ACTION.

§ 1. Object of the Investigation.

The principle assumed by the author as the ground of the second main principle, viz. that heat cannot of itself, or without compensation, pass from a colder to a hotter body, corresponds to everyday experience in certain very simple cases of the exchange of heat. To this class belongs the conduction of heat, which always takes place in such a way that heat passes from hotter bodies or parts of bodies to colder bodies or parts of bodies. Again as regards the ordinary radiation of heat, it is of course well known that not only do hot bodies radiate to cold, but also cold bodies conversely to hot; nevertheless the general result of this simultaneous double exchange of heat always consists, as is established by experience, in an increase of the heat in the colder body at the expense of the hotter.

Special circumstances may however occur during radiation, which cause the rays, instead of continuing in the same straight line, to change their direction; and this change of direction may be such, that all the rays from a complete pencil of finite section meet together in one point, and there combine their action. This can be accomplished, as is well known, by the use of a burning-mirror or burning-glass; and several mirrors or glasses may even be so arranged, that several pencils of rays from different sources of heat meet together in one point.