242 Prof. Kirchhoff on a New Proposition 
emitted from a body, whatever be its temperature, although the 
amount diminishes with the temperature. In proportion as a 
body radiates, it loses heat, and its temperature must sink unless 
this loss is made up. A body surrounded by substances of the 
same temperature undergoes no change of temperature. In this 
case the loss of heat caused by its own radiation is exactly com- 
pensated by the rays which surrounding substances give out, a 
part of which the body absorbs. The quantity of heat which 
this body absorbs in a given time must be equal to that which in 
the same time it emits. This holds good whatever the nature 
of the body may be; the more rays a body emits, the more of 
the incident rays must it absorb. The intensity of the rays which 
a body emits has been called its power of radiation or emission ; 
and the nuniber denoting the fraction of the incident rays which 
is absorbed has been called the power of absorption. The larger 
the power of emission a body possesses, the larger must its power 
of absorption be. 
A somewhat closer consideration shows that the relation be- 
tween the powers of emission and absorption for one temperature 
is the same fer all bodies. This conclusion has been verified in 
many special cases, both in the last ten years and in former 
times. The foregoing proposition requires, however, that all the 
rays of heat under consideration are of one and the same kind; so 
that these rays are not qualitatively so far different that one part 
of them are absorbed by the bodies more than another part ; for, 
were this the case, we could not speak of the power of absorption 
of a body, simply because it would be different for different rays. 
Now we have long known that there really are different kinds of 
heating rays, and that in general they are unequally absorbed by 
bodies. There are both dark- and luminous-heating rays; the 
former are almost all absorbed by white bodies, whilst the latter 
rays are thus scarcely absorbed at all. Indeed the variety of the 
rays of heat is even greater than the variety of the coloured rays 
of light. The rays of heat, the dark as well as the lumimous, 
are influenced in the same manner as the rays of light, by trans- 
mission, reflexion, refraction, double refraction, polarization, in- 
terference, and diffraction. In the case of the lummous rays of 
heat, it is not possible to separate the light from the heat; when 
one is diminished in a given relation, the other is diminished in 
the same ratio. This has led to the conclusion that rays of light 
and heat are essentially of the same nature ; that rays of light are 
simply a particular class of the heat-giving rays. The dark rays of 
heat are distinguished from the rays of light , Just as the differently 
coloured rays are distinguished from each other, by their period 
of vibration, wave- length, and refractive index. They are not 
visible because the media of our eyes are not transparent to them. 
