RADIATION AND ABSORPTION. 69 
an influence on the temperatures of the soil. He had been led 
to this by the beautiful experiments made by De Saussure, in 
1774, on some elevated summits of the Alps and in the adjacent 
plains, with a view to compare the relative intensities of solar 
heat. On that occasion* M. Fourier states in a precise man- 
ner one of the principles which have served me to establish the 
equations of equilibrium; only that he appeared to apply it 
merely to the solar action, supposing that this periodical action 
is the principal cause of the decrease of temperature of the 
atmosphere. 
On another side, M. Poisson, in his last work, has already 
shown that the upper strata of the atmosphere must necessarily 
be at a much lower temperature than the temperature of space: 
he has deduced this result, on one hand, from the numbers at 
which he has arrived to express the temperature of space, and 
on the other, from the mechanical conditions of equilibrium, 
which could not be fulfilled at the limits of the atmosphere if 
the air did not there experience a degree of cold sufficient to 
make it lose all its elasticity. This consequence, which might 
appear extraordinary when presented only as a mechanical ne- 
cessity, may perhaps now appear, if not more certain, at least 
more natural, since it results also from the laws of radiant heat, 
and since it is by this explained and referred to its real origin. 
17. If we now return to the conditions of equilibrium of dia- 
thermanous envelopes to examine the causes which may have 
influence upon their double absorbing power, we shall remark 
that the specific heat of the substance of these envelopes cannot 
change without the absorbing powers changing also in a certain 
relation. In fact, if around the globe we substitute for a given 
envelope another envelope of the same mass and the same mat- 
ter, which differs from it only in its capacity for heat, it is ex- 
tremely probable that the effects will be different, that these 
two envelopes will not take the same temperature, and that they 
will not determine equal accumulations of heat on the globe, 
even supposing that the relative values of the two absorbing 
powers remain the same in each of them. 
This simple remark, joined to some other considerations which 
cannot be developed here, has led me to admit that the absorbing 
powers of the same elastic fluid, considered as a diathermanous 
substance, are proportional to its mass and to its capacity for 
* Annales de Chimie, tome xxvii. p. 155. 
