REFLECTION. As the rays of light are reflected by polished surfaces, so it is found that the rays of caloric have the same property. The Swedish chemist Scheele discovered, that the angle of re flection of the rays of caloric is equal to the angle of incidence. This has been more fully established by Dr. Herschel. Some very interesting experiments were made by Professor Pictet of Geneva, which proved the same thing.
These experiments were conducted in the following manner. Two concave mirrors of tin, of nine inches focus, were placed at the distance of twelve feet two inches from each other. In the focus of the one was placed the bulb of a thermo meter, and in that of the other a ball of iron two inches in diameter, which was just heated so as not to be visible in the dark. In the space of six minutes the thermometer rose 22°. A similar ef fect was produced by substituting a light ed candle in place of the ball of iron. Supposing that both the light and heat acted in the last experiment, he interpos ed between the two mirrors a plate of glass, with the view of separating the rays of light f'rom those of caloric. The rays of caloric were thus interruired by the plate of glass, but the rays of light were not perceptibly diminished. In nine minutes the thermometer sunk 14° ; and in seven Minutes after the glass was removed, it rose about 12°. f le therefore justly con cluded, that the caloric reflected by the mirror, was the cause of the rise of the thermometer. He made another experi ment, substituting boiling water in a glass vessel in place of the iron ball ; and when the apparatus was adjusted, and a screen of silk which had been placed between the two mirrors removed, the thermometer rose 3° ; namely, from 47° to 50°. The experiments were varied by removing the tin mirrors to the distance of 90 inches from each other. The glass vessel with boiling water was placed in one focus, and a sensible thermometer in the other. In the middle space between the mirrors, there was suspended a common glass mirror, so that either side could be turn ed towards the glass vessel. When the polished side of this mirror was turned to wards the glass vessel, the thermometer rose only five-tenths of a degree ; but when the other side, which was darkened, was turned towards the glass vessel, the thermometer rose 3° 5'. And in another experiment performed, in the same way, the thermometer rose 3° when the polish ed side of the mirror was turned to the glass vessel, and 9° when the other side was turned. These experiments show clearly, that the rays of caloric are re flected from polished surfaces, as well as the rays of light. Transparent bodies have the power of refracting the rays of caloric as well as those of light. differ also in their refrangibility. So far
as experiment goes, the most of the rays of caloric are less refrangible than the red rays of light. The experiments of Dr. Herschel show, that the rays of ca loric, from hot or burning bodies, as hot iron, hot water, fires and candles, are re frangible, as well as the rays of caloric which are emitted by the sun. Whether all transparent bodies have the power of transmitting these rays, or what is the difference in the refractive power of these bodies, is not yet known.
The light which proceeds from the sun seems to be composed of three distinct substances. Scheele discovered, that a glass mirror held before the fire reflect ed 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. And if a plate of glass be held before a burning body, the rays of light are not sensibly interrupted, but the rays of ca loric 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 fa cility as those of light. And thus the rays of light and caloric may be separa ted. 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 these experiments, the different co loured rays were thrown on the bulb of a very delicate thermometer, and their heating power was observed. The heat ing power 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 refran, gible rays was least, and this power in creases as the refrangibility diminishes. The red ray, therefore, has the greatest heating power, and the violet, which is the most refrangible, the least. The il. luminating power, it has been already oh served, is greatest in the middle of the spectrum, and it diminishes4owards both extremities ; but the heating power, which is least at the violet end, increased 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. 'I he beat ing power of these invisible rays was great est at the distance of half an inch beyond the red ray, but it was sensible at the twice of one inch and a half. See OPTICS...