476 
MEASUREMENT OE CHEMICAL INTENSITY OF TOTAL DAYLIGHT. 
it was observed that they acquired a bright olive-green tinge, producing an effect as if 
they had been seen through yellow glass. This effect, it was remarked at the time, 
reached its maximum about l h 30 m , at which time it will he seen that the curves repre- 
senting the diminution of the direct and diffused chemical intensity are most widely 
separated from the mean curve representing the total chemical action. 
On a former occasion we communicated to the Royal Society the results of a series of 
Observations on the Chemical Intensity of Daylight, made at Moita, near Lisbon, at the 
level of the sea, during August 1867, from which we deduced the relation between the 
sun’s Altitude and the Photochemical Action of Total Daylight in a Cloudless Sky*. 
As Catania (lat. 37° 30') and Moita (lat. 38° 40') are nearly in the same latitude, it becomes 
interesting to compare the results obtained in December at the former place with those 
obtained for similar altitudes in August at the latter. The results of this comparison 
are seen in the following Table : — 
Table VII. 
Number of 
Chemical intensity. 
Mean altitude. 
observations. 
Sun. 
Sky. 
Total. 
Lisbon. 
Catania. 
L. 
C. 
L. 
C. 
L. 
C. 
L. 
C. 
O / 
1 30 
1 
o-ooo 
0-009 
0-009 
9 28 
7 
0-008 
0-044 
0-052 
9 51 
13 10 
15 
7 
0-000 
0-014 
0-038 
0-050 
0-038 
0-064 
19 41 
19 58 
18 
12 
0-023 
0-028 
0-062 
0-072 
0-085 
0-100 
24 46 
7 
0-049 
0-095 
0-144 
28 29 
14 
0-047 
0-108 
0-155 
31 14 
22 
0-052 
0-100 
0-152 
Figs. 2 & 3 give a graphical representation of these relations of chemical intensity as 
ordinates to the sun’s altitude as abscissae. The unbroken curve in fig. 2 shows the 
Catania observations, the dotted curve those made at Lisbon. In fig. 3 the observations 
of direct chemical intensity are represented by the broad lines, those made at Catania 
being distinguished by the unbroken curve. In all cases the positions of the experi- 
mentally determined points are given to show how closely they lie to the curves. In 
both cases it is evident that the relation between the solar altitude and the total chemical 
intensity is represented by a straight line, although the Catania observations slightly 
exceed by a constant difference those made at Moita, in conformity with the slight 
difference in latitude, and with the fact that the former determinations were made at 
a greater elevation above the sea-level. 
The Catania observations further confirm the fact which we then announced, that for 
altitudes below 50° the amount of chemical action effected by diffused daylight on a sur- 
face placed in the plane of the horizon is greater than that exerted by direct radiation, 
and also that at low altitudes (9° or 10°) direct sunlight is almost completely robbed of 
its chemically active rays. 
* Philosophical Transactions, 1870, p. 309. 
