66 M. POUILLET ON SOLAR HEAT, 
tion to the effects which the atmosphere produces either upon 
the heat of the sun, or upon the heat of the other heavenly 
bodies, which is in general designated under the name of heat of 
space, or planetary heat. 
With regard to the solar heat no doubt exists: we know that 
in traversing diathermanous substances it is less absorbed than 
the heat which is derived from different terrestrial sources, the 
temperature of which is not very high. It is true that we have 
been able to make the experiment only upon solid or liquid 
diathermanous screens; but we regard it as certain that the 
atmospheric stratum acts in the manner of screens of this kind, 
and that consequently it exercises a greater absorption upon the 
terrestrial than upon the solar rays ; we must also add, that this 
difference of action does not result, as is sometimes said, from 
the solar heat being luminous and the terrestrial heat obscure ; 
for, up to the present day, all that is known with regard to 
this subject leads us to think that there is neither warm light 
nor luminous heat: the rays of heat and of light may derive 
their origin from the same source, be emitted at the same time, 
and coexist in the same pencil of rays, but they preserve a dis- 
tinctive character, since, on the one hand, they may be separated 
the one from the other, and on the other hand, there is no In- 
stance of a ray of heat which has been transformed into a ray of 
light, nor of a ray of light, properly speaking, which has been 
transformed into a ray of heat. The inequality of absorption in 
question depends therefore on peculiar properties which the 
rays of heat assume when they are emitted by sources at a 
temperature more or less high, and these properties are only 
maintained, or perhaps developed more, when the temperature 
of the sources is sufficiently raised for them to emit, like the 
sun, light at the same time as heat. 
With respect to the heat of space, there is another distinction 
to be made : it must be considered in relation to its quantity and 
in relation to its nature. 
Considered in relation to its quantity, it is measured, as all 
other heat, by the effects which it produces ; that is to say by 
the quantity of ice which it can melt, or by the elevation of tem- 
perature which it would impart to a given quantity of water. 
It is upon this principle that M. Fourier has first shown that it} 
was necessary to take account of the heat of space in order to} 
explain the phenomena of the terrestrial temperatures ; and i 
