Messrs. Bunsen and Rosecoe’s Photochemical Researches. 67 
perpendicularly, of a column of hydrochloric acid 35°3 metres in 
height, assuming that the rays are extinguished by passing through 
an infinitely extended atmosphere of chlorine and hydrogen. By 
help of the above formula, it is also found that the sun’s rays, after 
they have passed in a perpendicular direction through the atmo- 
sphere 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 have been lost by extine- 
tion and dispersion in the atmosphere. The total chemical action 
emanating from the sun during each minute is therefore represented 
by a column of hydrochloric acid 35 metres in height, and having 
an area equal to the surface of a sphere whose diameter is the mean 
distance of the earth to the sun. Or the light which the sun 
radiates into space during each minute of time represents a chemical 
energy, by means of which more than 25 billions of cubic miles of 
chlorine and hydrogen may be combined to form hydrochloric acid. 
Tn like manner the amounts of chemical action have been calculated, 
which the sun’s rays, undiminished by atmospheric extinction, pro- 
duce at the surface of the chief planets. The first column of num- 
bers gives the mean distances of the planets from the sun, the second 
contains the chemical action expressed in light-metres. 
Mercury.......; 0°387 235°4 light-metres. 
eRUR hos, UVES OO OS Pr 
Earth.. 1-000 SEs - 
Mist tie ces tia, b'OZ4 5:2 5a 
* Te 5°203 1°3 fi 
PALIN hog ors 20 9°539 0:4 uy 
VRIES '< ate'ele 19°183 01 3 
Neptune........ 30°040 0:04 = 
By aid of the formula already given, the authors have been enabled 
to calculate the chemical action effected each minute by the direct 
sunlight, not only at different points on the earth’s surface, but at 
various heights above the sea’s level. Both these series of relations 
are tabulated, and graphically represented. On comparing the 
numbers and curves giving the action of the total diffuse light with 
those of the direct solar light, the singular fact becomes apparent, 
that from the North Pole to latitudes below that of Petersburg, 
the chemical action proceeding from the diffuse light is, throughout 
the day on the vernal equinox, greater than that effected by the 
direct sunlight; and that in lower latitudes, down to the Equator, 
the same phenomenon is observed, if not for the whole, still for a 
portion of the day, It is further seen, that for all places, and on 
every day when the sun rises to a certain height above the horizon, 
there is a moment at which the chemical action of the diffused light 
is exactly equal to that of the direct sunlight. The times at which 
these phases of equal chemical illumination occur can be calculated ; 
they can also be actually determined, by allowing the direct sunlight 
alone, and the whole diffuse daylight alone, to fall at the same time 
upon two pieces of the same sensitized photographic paper; the 
period at which both papers Reeee equally blackened, gives the 
2 
