Messrs. Bunsen and Roscoe’s Photochemical Researches. 65 
ing to which the chemical energy proceeding from the sun is diffused 
over the earth when the sky is unclouded. 
From a series of observations made on June 6, 1858, the relation 
between the amount of light optically measured falling from the 
whole sky, and the amount (taken as unity) which, at the same 
time, falls from « portion of zenith sky equal to >;4;th of the whole 
visible heavens, has been calculated for every degree of sun’s zenith 
distance from 20° to 90°; the results being tabulated, and also 
represented graphically. These numbers, multiplied by the che- 
mical light proceeding from the same portion of zenith sky for the 
same zenith distances, must give the chemical action effected by the 
whole diffuse daylight. ‘The amount of chemical light which falls 
from the zenith portion of sky is, however, the chemical brightness 
of that portion of sky. This chemical brightness has been deter- 
mined, by the chlorine and hydrogen photometer, on various days, 
and at different hours, when the sky was perfectly cloudless. A 
table contains the chemical action, expressed in degrees of light, 
which is effected on the earth’s surface by a portion of zenith sky 
equal in area to >;),,th of the whole visible heavens, under the cor- 
responding sun’s zenith distances from 20° to 90°. A curve repre- 
senting the relation between the action and the height of the sun, 
shows that although the single observations were made on different 
years and at different times of the year and day, they all agree 
closely amongst themselves, and hence another proof is gained of the 
slight effect which variation in the air’s transparency produces ; and 
it is seen that the total chemical action effected by the diffuse light 
of day may be represented as a function of the sun’s zenith distance. 
The numbers thus obtained have only to be multiplied by the 
corresponding numbers of the former table, in order to give the 
chemical action effected by the total diffuse light of day for zenith 
distances from 20° to 90°. A table and graphic representation of 
these numbers is given. Knowing the relation between the sun’s 
altitude and the chemical action, the chemical illumination effected 
each minute at any given locality at a given time may be calculated ; 
this calculation has been made for a number of places for each hour 
on the vernal equinox, tables and curves representing the alteration 
of luminous intensity with the height of the sun at these places being 
iven. 
: From these data it is possible also to calculate the action produced 
by the whole diffuse light, not only for each minute, but during any 
given space of time. For the following places the amount of che- 
mical illumination expressed in degrees of light which falls from sun- 
rise to sunset on the vernal equinox, is— 
Melville Island ............ 10590 
Heykiavik .....-cccpeaces . 15020 
St. Petersburg ...-.....0: 16410 
Manchester............---» 18220 
etd eer a. ee mite ne 19100 
TOR re 54a ee is ¢2 SOO ae 
CATO... . 21670 
Phil. May. S. 4. Vol. 19. No. 124, Jan. 1860. E 
