1342. The nature of light is only imperfectly known. The light which proceeds from the sun appears to be of two distinct kinds, viz. the rays of heat and the rays of light, and both of these produce a chemical action on plants. Scheele discovered that a glass mirror, held before the fire, reflected the rays of light, but not the rays of caloric; but when a metallic mirror was placed in the same situation, both heat and light were reflected. The mirror of glass became hot in a short time, but no change of temperature took place on the metallic mirror. This experiment shows that the glass mirror absorbed the rays of caloric, and reflected those of light; while the metallic mirror, suffering no change of temperature, reflected both. If a glass plate be held before a burning body, the rays of light are not sensibly interrupted, but the rays of caloric are intercepted; for no sensible heat is observed on the opposite side of the glass; but when the glass has reached a proper degree of temperature, the rays of caloric are transmitted with the same facility as those of light; and thus the rays of light and caloric may be separated. But the curious experiments of Dr. Herschel have clearly proved that the invisible rays which are emitted by the sun have the greatest heating power. In those experiments, the different coloured rays were thrown on the bulb of a very delicate thermometer, and their heating power was observed. The heating powers of the violet, green, and red rays were found to be to each other as the following numbers:-violet, 16.0; green, 22.4; red, 55.0. The heating power of the most refrangible rays was least, and this power increases as the refrangibility diminishes. The red ray, therefore, has the greatest heating power; and the violet, which is the most refrangible, the least. The illuminating power, it has been already observed, is greatest in the middle of the spectrum, and it diminishes towards both extremities; but the heating power, which is least at the violet end, increases from that to the red extremity: and when the thermometer was placed beyond the limit of the red ray, it rose still higher than in the red ray, which has the greatest heating power in the spectrum. The heating power of these invisible rays was greatest at the distance of half an inch beyond the red ray; but it was sensible at the distance of one inch and a half.
