PEOPESSOE BUN’SEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 925 
The chemical intensity of the sun’s rays at these various periods may be calculated by 
formula (14.). They are in the proportion of the numbers 1-002, 1-000 and 1-016. 
Although the differences between these numbers are but small, we have reduced all the 
observations to that chemical action which would have been observed if they had all 
been made at 12** 0“a.m. upon the day in question. The following Table contains the 
numbers thus reduced, the mean value having been taken of those observations which 
occur more than once : — 
Xo. 
True solar time. 
Position in the spectrum. 
Relative chemical 
action. 
1 
h m 
10 54 A.M. 
From GH to I 
62-7 
2 
10 58 a.m. 
From DE to E 
1-3 
3 
11 4 A.M. 
From C to T DE 
0-5 
4 
11 8 A.M. 
From Nj to -| QR 
18-9 
5 
11 13 a.m. 
From ^ hS to f ST 
2-1 
6 
11 41 A.M. 
From f ST to f UV 
1-2 
7 
11 47 a.m. 
From 1 N4Q to RS 
12-5 
8 
11 54 a.m. 
From-ilM, to N^... 
38-6 
9 
0 1 P.M. 
From Hj to ^ IMj 
55-1 
10 
0 4 P.M. 
From T GH to H 
60-5 
11 
0 16 P.M. 
From T EG to G 
28-4 
12 
0 20 P.M. 
From DE to F 
1-4 
13 
0 40 P.M. 
From G to 4 GH 
54-5 
The lines aaaa (fig. 19, Plate XL VII.) give a representation of the relative chemical 
action which the various parts of the spectrum, the rays of which have only j)assed 
through air and quartz, effect on the sensitive mixture of chlorine and hydrogen. It is 
seen that this action attains many maxima, of which the largest lies by y GH to H, and 
the next at I, and also that the action diminishes much more regularly and rapidly 
towards the red than towards the violet end of the spectrum. 
The sun, when it was employed for these experiments, was 35° 13' removed from the 
zenith. If the atmosphere were throughout of the density corresponding to 0"*-76 and 
0° C., the perpendicular height which, during our experiment, it would have possessed, is 
0-7494 
0-000095084 “■ 
7881 metres. 
The depth of atmosphere through which the rays had to pass in this experiment was, 
however, 
-— 3—^=9647 metres. 
d o 0 X o 
cos 
We have stated in one of our previous communications^, that the solar rays which 
at different hours of the day pass through the same column of chlorine, are altered in a 
very different manner. This shows that rays of different chemical activity are absorbed 
in very different ways by the air. The above results are therefore only applicable for 
sunlight which has passed through a column of air, measured at 0'^-76 and 0° C., of 9647 
metres in thickness. For rays which have to pass through a column of air of a ditferent 
length from this, the chemical action of the various constituents of the spectrum must 
* Philosophical Transactions, 1857, p. 617, &c. 
6 F 
MDCCCLIX. 
