912 PEOI^SSOE BUTsTSEN" AOT) DE. H. E. EOSCOE’S PHOTO-CHENnCAE EESEAECHES. 
which the tension of aqueous vapour is^=0“‘0154. Hence the value of h is found to be 
/^=0’00858 metre. 
By substitution of this number in the last formula, we have 
H=35-3 metres. 
From this we see that the sun’s rays, if they fell on the earth’s surface vdthout being 
weakened by passing through the atmosphere, would exert an action represented by 
35'3 light-metres; that is, they would effect in one minute a combination, on a surface 
upon which they fall perpendicularly, of a column of hydrochloric acid 35-3 metres 
in height, assuming that the rays are extinguished by passing through an infinitely 
extended column of the sensitive gas. 
By help of formula (14.) we find, moreover, — • 
That the sun’s rays, after they have passed in a perpendicular direction through the 
atmosphere to the sea’s level under a mean pressure of 0'76 metre, only effect an action 
of 14-4 light-metres, or that under these conditions, nearly two-thirds of their chemical 
activity has been lost by extinction and dispersion in the atmosphere. 
If we assume that the mean distance of the sun to the earth is r= 206 82 32 9 geogra- 
phical miles, and if we imagine the sun in the centre of a sphere whose radius =?’, the 
surface of this sphere is The light radiated from the sun upon this sphere would, 
if all the light were extinguished in the chlorine and hydrogen gas, produce in each 
minute a layer of hydrochloric acid of 36-3 metres, or 0 '004766 geographical mile in 
height. Hence it follows,-— 
That the light which the sun radiates into space during each minute of time, repre- 
sents a chemical energy, by means of which more than twenty-five and a half billions of 
cubic miles of chlorine and hydrogen may be combined to form hydrochloric acid. 
In a similar way the chemical action has been calculated which the sun’s rays, undi- 
minished by atmospheric extinction, produce at the surface of each of the eight chief 
members of our planetary system. The second column of the follomng Table (XVIH.) 
contains the mean distances of these planets from the sun ; the third column shows the 
chemical action represented in degrees of light eifected by the sun’s rays on the planet 
whose name is found in column I. ; and in column IV. this same action is given in light- 
metres. 
Table XVIII. 
I. 
II. 
IIL 
IV. 
Mercury ...... 
0-887 
2125-0 
light-metres. 
235-4 
Venus 
0-723 
608-9 
67-5 
The Earth ...... 
1-000 
318-3 
35-3 
Mars ............ 
1-524 
137-1 
15-2 
Jupiter 
6-2013 
11-8 
1-2 
Saturn ......... 
9-539 
3-5 
0-4 
Uranus 
19-183 
1-0 
0-1 
Neptune... 
30-040 
0-4 
0-04 
