PEOFESSOE BXJNSEK Am) DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 905 
In order to give a clear idea of the daily changes of chemical luminosity in such a piece of 
sky over which clouds passed, we have represented the observations by a curve (A, fig. 12, 
Plate XLVI.), in which the times are marked as abscissae, and the chemical actions 
expressed in units of light as ordinates. For the sake of comparison we have calculated 
the action which would have taken place on that day if the sky had been perfectly 
cloudless, and we have represented this action by the second curve, B. From 7^ a.m. to 
nearly noon the sky was covered with a layer of cloud of varying thickness, through 
which the sun but slightly shone. In the afternoon the clouds gradually disappeared, 
so that after 2 o’clock only light detached clouds passed over the zenith, and after 
about half-past 3 the sky became cloudless. These alterations are clearly seen on the 
curve, which shows that a thin film of cloud increased the quantity of light reflected 
from the portion of sky under observation to beyond four times the amount reflected 
from the same portion of sky when cloudless; and that the chemical action before noon 
rose much more rapidly, and was more irregular than the action in the afternoon when 
the clouds were gradually disappearing. It is seen too that, at 2*‘ 22“ and at 8^ 30™, 
the action attained two maxima limits, owing to the passage at those times of a bright 
white cloud through the zenith. 
These observations prove that the presence of a thin film of cloud increases the 
amount of chemical illuminating effect in the most striking manner ; we have, however, 
also convinced ourselves, by a large number of experiments, that a thicker layer of 
clouds, such as those which precede rain or thunder-storms, or dense mists or fogs, may 
so diminish the chemical light reflected from the portion of heavens under examination, 
that our instrument is unable to measure its amount. The clouds act, therefore, not 
meiely as collectors of atmospheric moisture, they also serve as mighty reflectors of 
light, legulating the supply of chemical rays, which are as necessary to the growth and 
well-being of plants as the Avarmth or moisture of the earth and air. 
After these considerations on the difiuse daylight, we may pass on to the measure- 
ment of the chemical action effected by the direct solar rays. 
III. CHEMICAL ACTION OF THE DIEECT SIINLIOHT. 
In order to obtain data for the determination of the theory of this action, the volume 
of hydrochloiic acid was measured, which was produced each minute in our instrument 
upon a cloudless day and at various zenith-distances of the sun, when the direct sun- 
light, diminished to a known extent, was allowed to fall perpendicularly over the whole 
surface of our insolation-vessel. For this purpose we directed the solar rays, by means 
of a SiLBEEMANN s heliostat, through a fine opening in a thin plate, and allowed the 
sun s image thus formed so to fall upon the insolation-vessel that every portion of the 
sensitive mixture of chlorine and hydrogen was equally exposed. The position in which 
this takes place is easily found by holding a piece of paper behind the apparatus, 
and then moving the instrument until the shadow of the insolation-vessel appears 
