water, throws the point of greatest heat on the orange; for the same reason, the crown and flint glass prisms transfer that point respectively to the red and to its limit. M. Melloni, observing that the maximum point of heat is transferred farther and farther towards the red end of the spectrum, according as the substance of the prism is more and more permeable to heat, inferred that a prism of rock-salt, which possesses a greater power of transmitting the calorific rays than any known body, ought to throw the point of greatest heat to a considerable distance beyond the visible part of the spectrum,—an anticipation which experiment fully confirmed, by placing it as much beyond the dark limits of the red rays as the red part is distant from the blueish green band of the spectrum. In all these experiments M. Melloni employed a thermomultiplier,—an instrument that measures the intensity of the transmitted heat with an accuracy far beyond what any thermometer ever attained. It is a very elegant application of M. Seebeck’s discovery of thermo-electricity; but the description of this instrument is reserved for a future occasion, because the principle on which it is constructed has not yet been explained. In the beginning of the present century, not long after M. Malus had discovered the polarization of light, he and M. Berard proved that the heat which accompanies the sun’s light is capable of being polarized; but their attempts totally failed with heat derived from terrestrial, and especially from non-luminous sources. M. Berard, indeed, imagined that he had succeeded; but, when his experiments were repeated by Mr. Lloyd and Professor Powell, no satisfactory result could be obtained. M. Melloni resumed the subject, and endeavoured to effect the polarization of heat by tourmaline, as in the case of light. It was already shown that two slices of tourmaline, cut parallel to the axis of the crystal, transmit a great portion of the incident light when looked through with their axes parallel, and almost entirely exclude it when they are perpendicular to one another. Should radiant heat be capable of polarization, the quantity transmitted by the slices of tourmaline in their former position ought greatly to exceed that which passes through them in the latter, yet M. Melloni found that the quantity of heat was the same in both cases: whence he inferred that heat from a terrestrial source is incapable of being polarized. Professor Forbes of Edinburgh, who prosecuted this subject with great acuteness and success, came to the same conclusion in the first instance; but it occurred to him, that, as the pieces of tourmaline became heated by being very near the lamp, the secondary radiation from them rendered the very small difference in the heat that was transmitted in the two positions of the pieces of tourmaline imperceptible. Nevertheless he succeeded in proving, by numerous observations, that heat from various sources is polarized by