PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 913 
From these numbers it is seen how lavish Nature has been in her distribution of 
chemical energy throughout the universe. The Earth receives but an infinitely small 
portion of this radiation, whilst Saturn and the more remote planets obtain so much less, 
that on these bodies the existence of any organic life at all similar to that enjoyed on 
our globe must be impossible. 
After these considerations touching the magnitude of the total chemical energy 
radiated from the sun, we may proceed to consider the conditions under which the 
small portion reaching the surface of our earth is distributed. 
Compared with the thermic actions of the sun’s rays, we immediately see an important 
difference. The heat, produced in the first instance by the absorption of the sun’s rays, 
is so irregularly distributed by radiation and by oceanic and atmospheric currents over 
the earth’s surface, that it rarely, if ever, is completely transformed at the time and at 
the place of its formation into real labour ; so that the simple law, according to which 
the thermic climate of a place is dependent upon its elevation above the sea’s level 
and the mean height of the sun, becomes completely obscured. The photo-chemical 
climate, on the other hand, is not subject to any such sweeping irregularities; for the 
chemical action which the sun effects upon a place, varying with its elevation above the 
sea and its geographical position, cannot be made to act at any point on the earth’s 
surface other than the one on which the rays directly fall. Hence the diffusion and 
arrangement of the photo-chemical energy follows a much more simple law than that 
exhibited in the distribution of heat on the earth’s surface. 
By means of formula (14.) the amount of chemical energy effected by the sun’s rays 
at a given time when the atmosphere is cloudless, can be calculated for any place whose 
geographical position is known, and for any height above the level of the sea. 
In the following Table (XIX.) is found the chemical action expressed in degrees of 
light which the direct sun’s rays effect at heights represented by barometric pressures 
of to 0“‘05, with varying sun’s zenith-distances from 90° to 0°. The highest hori- 
zontal division gives the zenith-distances, the first vertical column the atmospheric 
pressures, and the remaining columns the corresponding chemical actions. The curves 
(fig. 17, Plate XL\ II.) show graphically the dependence of the chemical illumination 
upon the barometric pressure. The abscissse denote sun’s zenith-distances, the ordinates 
the chemical action in light-degrees effected at these zenith-distances. With each curve 
the corresponding barometric pressure is given. From this we see how unequal, under 
otherwise similar circumstances, the chemical illumination of low-land and high-land 
must be, and how this illumination increases as the distance above the sea becomes 
higher in a greater ratio than corresponds to the diminution of atmospheric pressure. 
