1330. Two opinions are current respecting the nature of heat. By some philosophers it is conceived to be a peculiar subtile fluid, of which the particles repel each other, but have a strong attraction for the particles of other matter: by others it is considered as a motion or vibration of the particles of matter, which is supposed to differ in velocity in different cases, and thus to produce the different degrees of temperature. Whatever decision be ultimately made respecting these opinions, it is certain that there is matter moving in the space between us and the heavenly bodies capable of communicating heat; the motions of which are rectilineal: thus the solar rays produce heat in acting on the surface of the earth. The beautiful experiments of Dr. Herschel have shown that there are rays transmitted from the sun which do not illuminate, and which yet produce more heat than the visible rays; and Ritter and Dr. Wollaston have shown that there are other invisible rays distinguished by their chemical effects.