It has been observed, that animals in a medium hotter than the blood have a power of preserving themselves in the same temperature. In this case the heat is probably carried off by perspiration, while the blood ceases to receive, or give out, any heat; and Dr. Crawford finds, that when an animal is placed in a warm medium the colour of the venous blood approaches nearer to that of the arterial than when it is placed in a colder medium; and also, that it phlogisticates the air less than in the former case; so that in these circumstances respiration has not the same effect that it has in a colder temperature, in giving the body an additional quantity of heat; which is an excellent provision in nature, as the heat is not wanted, but, on the contrary, would prove inconvenient.
Another most important agent in nature, and one that has a near connexion with heat, is _light_, being emitted by all bodies in a state of ignition, and especially by the sun, the great source of light and of heat to this habitable world.
Whether light consists of particles of matter (which is most probable) or be the undulation of a peculiar fluid, filling all space, it is emitted from all luminous bodies in right lines.
Falling upon other bodies, part of the light is _reflected_ at an angle equal to that of its incidence, though not by impinging on the reflecting surface, but by a power acting at a small distance from it. But another part of the light enters the body, and is _refracted_ or bent _towards_, or _from_, the perpendicular to the surface of the new medium, if the incidence be oblique to it. In general, rays of light falling obliquely on any medium are bent as if they were attracted by it, when it has a greater density, or contains more of the inflammable principle, than the medium through which it was transmitted to it. More of the rays are reflected when they fall upon a body with a small degree of obliquity to its surface, and more of them are transmitted, or enter the body, when their incidence is nearer to a perpendicular.
The velocity with which light is emitted or reflected is the same, and so great that it passes from the sun to the earth in about eight minutes and twelve seconds.