PEOFESSOE BUNSEN AND DE. H. E, EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 919 
diffuse daylight ; division V. the chemical illumination effected by both the sun and 
diffuse light together ; and lastly, division VI. the height of the column of hydrochloric 
acid (at 0° and 0™-76), calculated according to formula (4.), which the total illumination 
of sun and daylight is able to combine during this one day. 
Table XXI. 
1 
II. 
III. 
IV. 
Y. 
VI. 
Melville Island 
Reykiavik 
Petersburg 
Manchester 
Heidelberg 
Naples 
Cairo 
1 Bombay 
Ceylon 
1 Borneo 
74 47 N.L. 
64 8 N.L. 
59 56 N.L. 
53 20 N.L. 
49 24 N.L. 
40 52 N.L. 
30 2 N.L. 
19 0 N.L. 
10 0 N.L. 
0 0 N.L. 
1196 
5964 
8927 
14520 
18240 
26640 
36440 
43820 
47530 
48940 
10590 
15020 
16410 
18220 
19100 
20550 
21670 
11790 
20980 
25340 
32740 
37340 
47190 
58110 
metres. 
1306 
2324 
2806 
3625 
4136 
5226 
6437 
From the numbers in division V. it is seen that the total chemical energy which the 
earth receives simiiltaneously fi’om the whole heavens and from the sun, varies compa- 
ratively slightly mth the latitude of the place. At Cairo the total chemical action 
is about five times, and at Heidelberg about twice as large as it is on Melville Island, 
which is only about 15° distant from the north pole. Notwithstanding this small 
difference, the highest position of the sun above the horizon on the day in question at these 
three places is very different; viz. at MehiUe Island, 15° 13'; at Heidelberg, 40° 36'; 
and at Caii’o, 59° 58'. The explanation of this remarkable relation is to be found in the 
large dispersive power of the atmosphere, which acts as a regulator of the photo- 
chemical processes occurring on the earth’s surface, modifying and lessening the great 
differences in the chemical illumination produced by the direct sunlight. This is 
plainly seen on reference to the numbers in dmsions III. and IV. The numbers repre- 
senting the chemical activity developed by sunhght alone m Melville Island, Heidelberg, 
and Cau’o, are nearly in the relation of I ; I5‘3 : 30’5, whereas the chemical actions 
brought about by diffuse dayhght at these places, when the amount of direct sunlight 
on Melville Island is taken as unity, are as 8‘9 : 16 : 18T. 
A comparison of columns HI. and IV. show moreover that, singularly enough, the 
amount of chemical energy effected by the diffuse daylight is, down to the latitude of 
Heidelberg, larger than that produced by the direct sunlight. At Heidelberg the action 
fi’om both sources is nearly equal ; at Petersburg that of the diffuse daylight is almost 
double of that of the sunlight ; and at Melville Island the effect produced by the 
former source is nearly ten times as large as that effected by the latter. 
6 E 2 
