ently
coloured rays. The greatest heat is exhibited in the red rays, the least
in the violet rays; and in a space beyond the red rays, where there is
no visible light, the increase of temperature is greatest of all.

From these facts it is evident that matter set in motion by the sun has
the power of producing heat without light, and that its rays are less
refrangible than the visible rays. The invisible rays that produce heat
are capable of reflection as well as refraction in the same manner as
the visible rays.

Rays capable of producing heat with and without light proceed not only
from the sun, but also from bodies at the surface of the globe under
peculiar agencies or changes. If, for instance, a thermometer be held
near an ignited body, it receives an impression connected with an
elevation of temperature; this is partly produced by the conducting
powers of the air, and partly by an impulse which is instantaneously
communicated, even to a considerable distance. This effect is called the
radiation of terrestrial heat.

The manner in which the temperatures of bodies are affected by rays
producing heat is different for different substances, and is very much
connected with their colours. The bodies that absorb most light, and
reflect least, are most heated when exposed either to solar or
terrestrial rays. Black bodies are, in general, more heated than red;
red more than green; green more than yellow; and yellow more than white.
Metals are less heated than earthy or stony bodies, or than animal or
vegetable matters. Polished surfaces are less heated than rough
surfaces.

The bodies that have their temperatures most easily raised by heat rays
are likewise those that are most easily cooled by their own radiation,
or that at the same temperature emit most heat-making rays. Metals
radiate less heat than glass, glass less than vegetable substances, and
charcoal has the highest radiating powers of any body as yet made the
subject of experiment.

Radiant matter has the power of producing chemical changes partly
through its heating power, and partly through some other specific and
peculiar influence. Thus chlorine and hydrogen detonate when a mixture
of them is exposed to the solar beams, even though the heat is
inadequate to produce detonation.

If moistened silver be exposed to the different rays of the solar
spectrum, it will be found that no effect is produced upon it by the
least refrangible rays which occasion heat